Add AnchorTagHelper.

- Added a TagHelper that targets the <a> tag and allows users to do the equivalent of Html.ActionLink or Html.RouteLink.

#1247
agreed general rule is that MVC tag helpers should simply not overwrite what user (or an earlier tag helper in line) specified. so, everywhere else, the `IHtmlGenerator` returns a `TagBuilder` with attributes that might not be copied to the `TagHelperOutput` because they are already there -- an no error results. there's nothing special here and it should follow the general rule. that is, get rid of this entire `if` block