Add AnchorTagHelper.

- Added a TagHelper that targets the <a> tag and allows users to do the equivalent of Html.ActionLink or Html.RouteLink.

#1247
reword last sentence for clarity: `Throws an <see cref="InvalidOperationException"/> if "href" attribute is provided and <see cref="Action"/>, <see cref="Controller"/>, or <see cref="Route"/>  are non-<c>null</c>.` (you left out the last bit of my reordered comment).