Add abspaths template function.

Prepends site URL to paths starting with /, for example,
if site url (in site.yml) is http://example.com, content

 .Content | abspaths

where .Content is:

  <img src="/some/pic.png">

will become:

  <img src="http://example.com/some/pic.png">

This is useful for feeds, because some feed readers don't
understand Atom's xml:base (e.g. FeedBurner's email output).