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
In addition to above comments (which apply to the rest of this `enum`), this one should emphasize that any tag helpers (and HTML helpers?) that apply to children of this helper will execute before it.
Somewhat separately is this the only behaviour in which this tag helper can _examine_ the HTML generated for its children?  Either way this