Super sexy rewrite

Create a custom class that can be used by either the tag or the attribute!
`Collection`s have `Document`s, only some of which should be written.

- https://github.com/jekyll/jekyll/blob/master/lib/jekyll/site.rb#L427-L429
- https://github.com/jekyll/jekyll/blob/master/lib/jekyll/document.rb#L250-L257
- https://github.com/jekyll/jekyll/blob/master/lib/jekyll/collection.rb#L127-L133