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
`_tagHelperDescriptorResolver` is initialized from a constructor parameter lacking `[NotNull]`.  either put lines 144 through 149 into an `if` block conditional on `_tagHelperDescriptorResolver != null` or add that `[NotNull]` to the constructor.