Added 'in' and 'intersect' as template functions

This pull request adds support for 'in' and 'intersect' template functions.  My motivation for developing this functionality was to be able to develop a 'similar posts' block which listed posts similar to ANY of the current pages tags.

Here is an example implementation in a `single.html` template:
```
<div class="panel panel-default">
    <div class="panel-heading" style="padding: 2px 15px;">
        <h4>Similar Posts</h4>
    </div>
    <div class="panel-body">
        <ul>
        {{ $page_link := .Permalink }}
        {{ $tags := .Params.tags }}
        {{ range .Site.Recent }}
            {{ $page := . }}
            {{ $has_tags := intersect $tags .Params.tags | len | lt 0 }}
            {{ if and $has_tags (ne $page_link $page.Permalink) }}
                <li><a href="{{ $page.Permalink }}">{{ $page.Title }}</a></li>
            {{ end }}
        {{ end }}
        </ul>
    </div>
</div>
```

Here is a simple set of tests to show how it works for different types: http://play.golang.org/p/thF-OCWclj