Add header identifier

The idea of this is to make every h1..h6 tag have an id so more savy
users can link to specific page sections. 
Story: https://www.pivotaltracker.com/story/show/73290726

This processor adds an id to any h* tag which doesn't already have one.
ID's are generated from the heading text prefixed with 'heading_'. Duplications
are avoided by appending '_2' or an iteration of when necessary.

This test feels like it’s testing multiple scenarios at once: how about splitting it out into one test for the repeated headings and one for the “some heading” clash?