Tag helpers registration system

- This registration system accepts tag descriptors that it then uses to feed requests for tag helpers.
- Added some infrastructure pieces that are used to build up valid tag helper descriptors.

#70
The tag helper author definitely will limit what a user can type in a given HTML attribute.  In the instance of a "string" for v1 we will project into quotes which will then restrict the use of variables, expressions etc.  We will loosen this in the future but for v1 string's and char's somewhat alter the system.