Addressed code review comments
Moved TagHelperOutput methods to extension methods.
Add FormTagHelper.

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

#1246
Addressed code review comments.
Made form utilize new copy helpers.
Add error to FormTaHelper when cannot determine action.
Modified methods in FormTagHelper to use new extension methods.
Added a conditional AntiForgery.