[Design] Add FormTagHelper.

- Added the FormTagHelper.
- Utilized the IHtmlGenerator to share base functionality with the HTMLHelper counterparts.

#1246
confusing explanation. for one thing, nothing is being "re-added" since these entries have never been in `TagHelperOutput`.  for another, it looks as if the real intent is to maintain whatever _case_ the user typed for the attribute names and this comment doesn't mention case.