[Design] 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
Decided to go with the `CreateTagHelperDescriptorResolver` design.  The alternative would be take the `DecorateTagHelperDescriptorResolver` approach.  Only issue with the "Decorate" approach is that there'd be nothing to decorate on the resolver.