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.

It feels like this should be a set to me.  Some pages will have a lot of ids; checking for existence each time a new heading is found is O(n*n) with a list, but O(n) with a set (assuming, slightly dodgily, that a set is O(1) for lookup; but it's something approximating that).