Add heading tag helper to Dough

@benbarnett 

Works now. Huzzah! :shipit:  
The reason that I made the question on the PR is that  if is public method, Imho it's better to add documentation to the method, with a block and without, and other developers could see the possibilities of this method. :-)
If don't know how can you add some rdoc comments, we can pair on that.