Form Helper

This is a WIP spike, please feel free to comment or suggest changes. An end goal for
me to is have something like
[SimpleForm](https://github.com/plataformatec/simple_form/) so hopefully
@josevalim will have some thoughts.

This will also rely heavily on the `Resource` protocol discussed in #320.
In my opinion, the tag helper should always be raw. We could provide
link_to or link_tag if someone wants a better behaviour out of the box.