Add extensions for TagHelperOutput to copy and merge from TagBuilder.

- Added tests to validate functionality of all TagHelperOutputExtension methods.

#1319
Add FormTagHelper.

- Added the FormTagHelper.
- Utilized the IHtmlGenerator to share base functionality with the HTMLHelper counterparts.
- Added tests to validate FormTagHelper functionality.

#1246
Add AnchorTagHelper.

- Added a TagHelper that targets the <a> tag and allows users to do the equivalent of Html.ActionLink or Html.RouteLink.
- Added tests to validate AnchorTagHelper functionality.

#1247
Add ValidationMessageTagHelper.

- Added tests to validate ValidationMessageTagHelper functionality.

#1250