Decouple post navigation from article tag

<article>

	<header>{{ partial "title" . }}</header>

	{{ .Content }}

</article>

<nav class="no-print post-nav">

{{ if .PrevInSection }}

	<a class="prev-post" href="{{ .PrevInSection.Permalink }}">

	</a>

{{ end }}

</nav>

</article>

{{ partial "related" . }}

{{ if in .Site.Params.DisqusSections .Section }}

	{{ partial "disqus" . }}