Add @addtaghelper directive. 

- Also added some infrastructure pieces for it such as the ITagHelperDescriptorResolver and the default implementation TagHelperDescriptorResolver which will be filled out in a later commit.
- Reworked some extensibility points to allow accessibility of the descriptor resolvers per offline discussions.
- Fixed existing tests to work with new RazorParser.
- Validated directive syntax tree creation, errors and code generation.

#111
3rd bullet point: Yes the code generator is not created until the user fulfills the quotation mark requirement (not good) I see your point.  

I'll modify to still create the code generator so the user at least has coloring once they've added their first quotation mark.