Capture handling improvemements

This does a few different things:  

* rename `CaptureTag` to the more generic `Capture` since it will be doing more than just tag handling.
* add `erb_outvar_name` and `erb_outvar` for accessing outvar info
* add `capture` that takes "captured content" and adds it to the outvar
* update `capture_tag` to use these formal methods internally (and implicitly test them)
* add `capture_render` which proxies a Deas template `render` call, captures its output, and adds it to the outvar.

This is all to provide a more rich capture API to allow users to do complex renders in their ERB templates.

@jcredding ready for review.