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.
If you make this line be this:

    do: build_attrs(tag, t, [[{:"data-#{k}", v}]|acc])

Then there is no need for calling flatten (the same for some clauses below).