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
could user be projected to `|` in `var __tagHelperDirectiveSyntaxHelper = "|";` when `!_initializedTagHelperDirectiveSyntaxHelper`?  doesn't matter that much but type of `__tagHelperDirectiveSyntaxHelper` isn't relevant since the user is typing inside a `string` literal.