Configurable namespaced inputs

This is continuation of #1043

* it simplifies defining input/action namespaces of form builder

Extracted custom namespace fix to #1065 

I have no idea why it is red, but will check it later.
For example activeadmin has custom inputs that have suffix in their name. So it is used like https://github.com/mikz/activeadmin/commit/3d933a3.
It could also be done by subclassing finder and overriding a class method.