Add AnchorTagHelper.

- Added a TagHelper that targets the <a> tag and allows users to do the equivalent of Html.ActionLink or Html.RouteLink.

#1247
+1 this is another case where, like "controller" and "route-value" in the `<form/>` helper, we should be consistent. I agree we shouldn't second-guess the users's intent when they force the tag helper to no-op.