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
why must a non-`null` `TagHelperDescriptorResolver` be passed to the parser?  are you saying the host can disable tag helper resolution but not turn off the extra parsing pass?