[Design] Add AnchorTagHelper.

- Added a TagHelper that targets the <a> tag and allows users to do the equivalent of Html.ActionLink or Html.RouteLink.

#1247
Is this is coming from the base TagHelper in a further change (that just doesn't show up yet) ?

If so it should just be straight methods on the base class, instead of an unnecessary indirection property.

Also the name PullPrefixedAttributes is meaningless