Add `GenerateAntiForgery()` to `IHtmlGenerator`
Add copy helpers for TagHelperOutput from TagBuilder.

- Added a new test project for tag helper specific code.
- Tested out Merge and MergeAttributes.

#1319
Addressed code review comments.
Add FormTagHelper.

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

#1246
Addressed code review comments.