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>

This "hack" to the navbar.html file should allow open links that are menu items in new tabs while keeping local links behave the same.

This file should be saved under your own local hugo project layouts directory (if you don't have such a directory create it and put this file there).

.
├── archtypes
├── content
├── data
├── layouts
|   └── partials
|       └── navbar.html (this file)
├── static
├── themes
└── config.toml

This should override the theme default behaviour, until there is a stable solution from Hugo

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 }}