Add suport for remote data sources for code fragments

[Crell]

We needed to make this addition for our own use, so I figure it's worth offering back.

Basically, this integrates the functionality of the codesnippet plugin into this plugin. If you add a url attribute to a tag, then the body gets replaced with the result of that URL. The intent is to have "live code" that we can run and make sure it always works, and them embed that into documentation knowing that it works.