Support "proxy" element

[timriley]

Make it possible to e.g. have a standard-rendered form but with a place we can hook in some custom markup (e.g. for a custom-built field or similar). This would make it possible to continue to use formalist for forms that do require some customisation.

I'd see it working something like this...

In the form definition:

proxy :my_custom_field

In the markup, somewhere on the same page as where the form rendering is happening:

<div data-formalist-proxy="my_custom_field">
  <!-- ALL THE CUSTOM MARKUP HERE -->
</div>

The formalist renderer would know to look for this and insert it into the right place on the form.

The challenge here would be working out a way to get the actual data for the element into that custom markup. @makenosound — do you have any ideas?