Add FormTagHelper.

- Added the FormTagHelper.
- Utilized the IHtmlGenerator to share base functionality with the HTMLHelper counterparts.

#1246
typo in "specified"

nits:
- "URL-based" (add hyphen)
- why mention "specified" at all? that should be implied from "URL-based"
- not sure about talking in terms of `FormTagHelper`; user seeing this message is writing a `<form/>` element