Form: allow _findFields to be overridden, provide 'after' event

DO NOT MERGE: this is bikeshed material for https://github.com/behance/be.net/pull/8222
I'd store `Object.keys(meta.data)` into a var, so it can be reused here: https://github.com/behance/BeFF/pull/53/files#diff-6b3680efaa54f44dae3bf140cfd880ecR161.

This would also allow you to then use a forEach and avoid the `hasOwnProperty` guard.