Add ITagHelperActivator and concrete implementation.

- The TagHelperActivator enables dependency injection via properties and allows access to the ViewContext.
- This replaces the ICanHasViewContext mechanism that we had in place before.
- Added tests and fixed up existing to work with new format for providing ViewContext.

#1258
Took the existing `DefaultViewComponentActivator` and cleaned it up a bit to work for what we needed for `TagHelper`s.