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
"body of the generated tag" is confusing since the InnerHtml will be part of the generation (eventually).  Might be better to describe the tag helper's inner HTML relative to "anything generated for the children of the target tag."  Also reference specific properties -- some combination of `<see cref="TagBuilder.InnerHtml"/>` and the correct member of the helper itself.