Add AnchorTagHelper.

- Added a TagHelper that targets the <a> tag and allows users to do the equivalent of Html.ActionLink or Html.RouteLink.

#1247
looks inconsistent and I'd like to understand the different between `action`, `controller`, and `route` versus `fragment`, `host`, and `protocol`.  why throw if `action` is specified but restore `protocol`?  none of these 6 are HTML 5 attributes.