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.
Guess it is a feature. It simplifies:
```ruby
f.action :custom, as: :custom
# to
f.action :custom
```
and prevents weird exception like `can't find Formtastic::Actions::NilClassAction` when `:as` was not passed and it was custom action.