Support .erb files and pass in config values as tpl var

This allows a project to have .erb files such that:

* The `.result` of 'test.html.erb' will be saved to `test.html` 
* Adds the object from the `YAML::load_file` of hologram_config.yml to our template binding

This should allow us to do things like create an index page of all pages and sub sections for the style guid, or #78. Additionally, this should take care of #73. You could list your js and css includes in the config file and have a _header.html that will generate the link/script tags.

I believe I may have mixed a couple changes across PRs but we can work those out through the merges

cc @Yago31