Support for Custom Form Actions

[bovorasr]

e.g.

When rendering a form, I would like the ability to provide custom actions that can operate on this form:

where submit, cancel, save, delete, etc. are objects describing the actions in some way.

[chriswxyz]

strongback/src/form/form.tsx

Line 44 in dedf33b

<div>

44-50 could be provided by an optional func that receives the props that section is using (pristine, submitting, reset).