This is a hack to allow menu items with external links to be open in a new tab

navbar.html

<!-- Header -->
<header id="header">
    {{ if or .IsHome (not .Site.Params.DynamicTitles) }}
    <h1><a href="{{ "/" | relURL }}">{{ .Site.Params.navbarTitle }}</a></h1>
    {{ else if ne .Section "" }}
    <h1><a href="{{ "/" | relURL }}">{{ .Section }}</a></h1>
    {{ else }}
    <h1><a href="{{ "/" | relURL }}">{{ .Title }}</a></h1>
    {{ end }}
    
    <nav class="links">
        <ul>
            {{ range .Site.Menus.main }}
            {{ $url := urls.Parse .URL }}
            {{ $baseurl := urls.Parse $.Site.Params.Baseurl }}
            <li>
                <a href="{{ .URL }}" {{ if ne $url.Host $baseurl.Host }}target="_blank" {{ end }}>
                    <i class="{{ .Identifier }}">&nbsp;</i>{{ .Name }}
                </a>
            </li>
            {{ end }}
        </ul>
    </nav>
    <nav class="main">
        <ul>
            {{ if $.Scratch.Get "shareNav" }}
            <li id="share-nav" class="share-menu" style="display:none;">
                <a class="fa-share-alt" href="#share-menu">Share</a>
            </li>
            {{ end }}
            <li class="search">
                <a class="fa-search" href="#search">Search</a>
                <form id="search" method="get" action="//google.com/search">
                    <input type="text" name="q" placeholder="Search" />
                    <input type="hidden" name="as_sitesearch" value="{{ .Site.BaseURL }}">
                </form>
            </li>
            <li class="menu">
                <a class="fa-bars" href="#menu">Menu</a>
            </li>
        </ul>
    </nav>
</header>

<!-- Menu -->
<section id="menu">
    
    <!-- Search -->
    <section>
        <form class="search" method="get" action="//google.com/search">
            <input type="text" name="q" placeholder="Search" />
            <input type="hidden" name="as_sitesearch" value="{{ .Site.BaseURL }}">
        </form>
    </section>
    
    <!-- Links -->
    <section>
        <ul class="links">
            {{ range .Site.Menus.main }}
            {{ $url := urls.Parse .URL }}
            {{ $baseurl := urls.Parse $.Site.Params.Baseurl }}
            <li>
                <a href="{{ .URL }}" {{ if ne $url.Host $baseurl.Host }}target="_blank" {{ end }}>
                    <h3>
                        <i class="{{ .Identifier }}">&nbsp;</i>{{ .Name }}
                    </h3>
                </a>
            </li>
            {{ end }}
        </ul>
    </section>
    
    <!-- Posts List -->
    <section id="recent-posts">
        <div class="mini-posts">
            <header>
                <h3>Recent Posts</h3>
            </header>
            {{ $.Scratch.Set "recentPosts" .Site.Pages }}
            
            {{ with .Site.Params.postAmount.sidebar }}
            {{ $.Scratch.Set "postLimit" . }}
            {{ else }}
            {{ $.Scratch.Set "postLimit" 5 }}
            {{ end }}
            
            {{ range first ($.Scratch.Get "postLimit") (where ($.Scratch.Get "recentPosts") "Type" "post") }}
            <article class="mini-post">
                <header>
                    <h3><a href="{{ .RelPermalink }}">{{ .Title }}</a></h3>
                    {{ $.Scratch.Set "dateType" .Date }}
                    <time class="published" datetime=
                    '{{ ($.Scratch.Get "dateType").Format "2006-01-02" }}'>
                    {{ ($.Scratch.Get "dateType").Format "January 2, 2006" }}</time>
                </header>
                {{ .Render "featured" }}
            </article>
            {{ end }}
            
            {{ if ge (len (where .Site.Pages "Type" "post")) ($.Scratch.Get "postLimit") }}
            <a href=
            {{ with .Site.Params.viewMorePostLink }}
            {{ . }}
            {{ else }}
            "/post/"
            {{ end }}
            class="button">View more posts</a>
            {{ end }}
        </div>
    </section>
    
    <!-- Actions -->
    <!--
        <section>
            <ul class="actions vertical">
                <li><a href="#" class="button big fit">Log In</a></li>
            </ul>
        </section>
    -->
</section>

Here is the equivalent for those modifying header.html (layouts/partials/header.html)

        {{ with .Site.Menus.main}}
          {{ range sort . }}
          {{ $url := urls.Parse .URL }}
          {{ $baseurl := urls.Parse $.Site.Params.Baseurl }}
            <li class="navigation-item">
              <a class="navigation-link" href="{{ .URL | relLangURL}}" {{ if ne $url.Host $baseurl.Host }}target="_blank" {{ end }}>{{ .Name }}</a>
            </li>
          {{ end }}
        {{ end }}