Sitecore 8 replacement for raw:AddCustomField from Sitecore 7

CustomIndexField

namespace NavArts.Sitecore.Search.ComputedFields
{
    using System;
    using System.Linq;
    using System.Xml;
    using global::Sitecore.ContentSearch;
    using global::Sitecore.ContentSearch.ComputedFields;
    using global::Sitecore.Data;
    using global::Sitecore.Data.Fields;
    using global::Sitecore.Diagnostics;

    /// <summary>
    /// Adds a Sitecore Field to the Index. 
    /// </summary>
    public sealed class CustomIndexField : IComputedIndexField
    {
        /// <summary>
        /// Gets or sets the FieldName from the Index Configuration File
        /// </summary>
        public string FieldName { get; set; }

        /// <summary>
        /// Gets or sets the ReturnType from the Index Configuration File
        /// </summary>
        public string ReturnType { get; set; }

        /// <summary>
        /// Gets or sets the SitecoreFieldName
        /// <para>Controlls what field to pull data out of</para>
        /// </summary>
        public string SitecoreFieldName { get; set; }

        /// <summary>
        /// Creates a new instance of the CustomIndexField with a null XmlNode
        /// </summary>
        public CustomIndexField() : this((XmlNode)null)
        {
        }

        /// <summary>
        /// Creates a new instance of the CustomIndexField with the passed in configuration
        /// </summary>
        public CustomIndexField(XmlNode configurationNode)
        {
            this.Initialize(configurationNode);
        }

        /// <summary>
        /// Check to see if the Indexed Item is a Standard Values Item
        /// </summary>
        /// <param name="indexable">The Indexed Item</param>
        /// <returns>True if the Item being indexed is a Standard Values Item</returns>
        public object ComputeFieldValue(IIndexable indexable)
        {
            var item = (SitecoreIndexableItem)indexable;

            if (item != null
                && item.Item != null
                && !String.IsNullOrEmpty(this.SitecoreFieldName))
            {
                var field = FieldTypeManager.GetField(item.Item.Fields[this.SitecoreFieldName]);

                if (field != null && field.InnerField != null)
                {
                    var multilistField = field as MultilistField;

                    if (multilistField != null)
                    {
                        if (multilistField.TargetIDs.Any())
                        {
                            return multilistField.TargetIDs.Select(id => ShortID.Encode(id).ToLower());
                        }
                    }

                    var lookupField = field as LookupField;

                    if (lookupField != null)
                    {
                        if (lookupField.TargetItem != null)
                        {
                            return ShortID.Encode(lookupField.TargetID).ToLower();
                        }
                    }

                    if (field.InnerField.HasValue)
                    {
                        return field.InnerField.Value;
                    }
                }
            }

            return null;
        }

        /// <summary>
        /// Sets the <see cref="SitecoreFieldName"/> from the configuration elements
        /// </summary>
        /// <param name="configurationNode">Passed in configuration</param>
        private void Initialize(XmlNode configurationNode)
        {
            if (configurationNode == null)
            {
                Log.Error("<name> configuration error: cannot be found.", this);
                return;
            }

            if (configurationNode.ChildNodes.Count > 0)
            {
                var xmlNode = configurationNode.SelectSingleNode("name");

                if (xmlNode == null || String.IsNullOrEmpty(xmlNode.InnerText))
                {
                    Log.Error("<name> configuration error: cannot be empty.", this);
                    return;
                }

                this.SitecoreFieldName = xmlNode.InnerText;
            }
        }
    }
}

NavArts.Example.LuceneConfiguration.config

<?xml version="1.0" encoding="utf-8" ?>
<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <contentSearch>
      <indexConfigurations>
        <defaultLuceneIndexConfiguration>
          <!-- Examples:
            Adding multiple fields of the same name to the same document
            The <name> sets the Sitecore field name of the item to set
            -->
          <fields hint="raw:AddComputedIndexField">            
            <field fieldName="tags" type="NavArts.Sitecore.Search.ComputedFields.CustomIndexField,NavArts.Sitecore">
              <name>tags</name>
            </field>            
            <field fieldName="tags" type="NavArts.Sitecore.Search.ComputedFields.CustomIndexField,NavArts.Sitecore">
              <name>topics</name>
            </field>
          </fields>
          <!-- Examples:
            Map our 'tags' computed fields to an index field
            -->
          <fieldMap type="Sitecore.ContentSearch.FieldMap, Sitecore.ContentSearch">
            <fieldNames hint="raw:AddFieldByFieldName">
              <field fieldName="tags"     storageType="YES" indexType="TOKENIZED"    vectorType="NO" boost="1f" type="System.String" settingType="Sitecore.ContentSearch.LuceneProvider.LuceneSearchFieldConfiguration, Sitecore.ContentSearch.LuceneProvider">
                <Analyzer type="Sitecore.ContentSearch.LuceneProvider.Analyzers.LowerCaseKeywordAnalyzer, Sitecore.ContentSearch.LuceneProvider" />
              </field>
            </fieldNames>
          </fieldMap>
        </defaultLuceneIndexConfiguration>
      </indexConfigurations>
    </contentSearch>
  </sitecore>
</configuration>