Allow partials to work as decorators

[bep]

This is a feature that I have been wanting, but always assumed would not be possible with Go templates. Now I think that it could be implemented.

Example:

single.html:

<div>
  {{ with partial "boldtitle.html" . }}
      {{ .Title }}
  {{ end }}
</div>

boldtitle.html:

<h1 class="bold">{{ .Inner }}</h1>

The above example is obviously very simple, but I suspect people can imagine their own us of this.