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
```C#
if (!_registrations.TryGetValue(descriptor.TagName, out var value))
{
    value = new HashSet<TagHelperDescriptor>();
   _registrations.Add(value);
}

_registrations.Add(descriptor);