Add ContentFilter Globs

[vmadman]

Sometimes I only want to run Grits on a subset of the content, especially when testing or developing Grits.

I'd like to be able to reduce the rendering to content that matches Globs (or perhaps, regex, or some other pattern), in order to reduce the amount of rendering that Grits will do during the operation, so that it can run much more quickly.

Other Notes:

• Content Filters should be specifiable as CLI arguments
• Content Filters should be specifiable within a config file
• Consider making other things, such as helpers, etc, filterable in the same way.
• Also consider adding .gritsignore files