If a section key returns a function, it will be called
and passed both the unrendered block of text and a renderer
convenience function.

Given this JS:

  "name": "Tater",
  "bolder": function() {
    return function(text, render) {
      return "<b>" + render(text) + '</b>'
    }
  }

And this template:

  {{#bolder}}Hi {{name}}.{{/bolder}}

We'll get this output:

  <b>Hi Tater.</b>

As you can see, we're pre-processing the text in the block.
This can be used to implement caching, filters (like syntax
highlighting), etc.

You can use `this.name` to access the attribute `name` from
your view.

This reverts commit 738de0bcae.

Conflicts:

	mustache.js

62efd014fc (comments)

Thanks Nikita!

Changing the delimiter and use the new one on the
same line was broken. Fixed that and extended the
test suite.