Add renderForm prop to Form

[danielweinmann]

The renderForm function would receive the same arguments as children + Form (which would be the value of component prop passed either to the form or to createForm) + fetcher (which would be the value of the fetcher prop).

It would be called whenever there are no children passed to the form, just like renderField is called when there are no children passed to Field.

That would allow us to customize the default markup of the forms, including custom pending UI like the one described in #37.

[gustavoguichard]

Wouldn't renderForm be a better name then?
I think it'd be clearer

[danielweinmann]

Maybe. It felt redundant to me since it will be the render prop of Form. But maybe redundancy is good in this case :)

[diogob]

I think we should start investigating a generic API for markup customization, that would preclude adding props every time we want to customize some new element. Perhaps exposing mapper functions that could be composed and passed around or set as part of the default form?