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
my comment wasn't well placed.  I meant your base method has `string` and `IEnumerable<TagHelperDescriptor>` parameters but then you add `string` and `bool` parameters between them.  that's not normal.