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
If the attribute value type is "string" a user would not be able to type ```@(string)Foo``` because they'd be projected inside of C# quotes.  The only way the user can do what you're referring to is to have a RazorExpression in which case this code path would not surround the value in quotes.