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
thought we'd agreed object creation was not required in most cases?  that is, only properties deriving from `TagHelperExpression` should need to be `new`ed up.  for all the other cases (strings, integers, complex types, enumerations, ...), can spit out almost exactly the text the user typed.  ("almost" because `@` signs need to be elided and literal characters and strings need to be quoted and escaped.)