Created docs outlining Front Matter Variables

#42 

Added `/_data/front-matter.yml` that feeds the documentation page.  If you create a new variable you should add it to the list so it auto-populates the docs.  The docs won't be included in a standard build.

To view the docs you need to run jekyll with the `--unpublished` flag and visit /docs.

I couldn't figure out how to get a list of flags jekyll was run with in order to add a "Docs" menu item, although it might be possible.