Render Liquid in CoffeeScript files

Closes https://github.com/jekyll/jekyll/issues/2792. Is it really this simple?

Incidentally, it seems a bit odd for `render_with_liquid?` to always return `true`, but I guess it's nice to keep the function around just in case we need to disable Liquid for some other file type in the future.

/cc @gjtorikian and @parkr for review :bow: