Extract #format from Tag#to_html.

The HTML rendering is now done in two parts:

1. Tag#format method is called, passing the DocFormatter.
   This is where Markdown to HTML conversion happens.

2. Tag#to_html method is called, which must just return HTML.
   No more can formatter be used inside #to_html.

This simplified the passing of DocFormatter instance to Tags -
no more do we need to inject the formatter inside Tag classes,
instead we can just pass it as a parameter.  With this we remove
the single remaining state object from Tag class.