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
To make full use of that approach we'd have to make the class static which would restrict us from deriving from this default implementation and making use of the ```IsSet``` method, ```GenerateValue``` etc.  That would also restrict users from deriving from our types if they wanted to build their own generators.