[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
I know it's just a design PR, but: recommend passing in `href` as a parameter to the string. And also recommend not pluralizing the `{0}s` in the second sentence, and instead saying `A {0} with a specified...`