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
(let's help the children :smiley_cat:) add comments referring reader to code in `CSharpCodeParser.TagHelperDirective()` that ensures `LookupText` is _not_ surrounded by the quotes.  Might also be worth mentioning this restores quotes that code requires then removes.