Custom inputs namespaces

This add the custom inputs namespace feature that was originally created in this PR #1081.

This PR fixes the custom input class lookup on previous versions of ruby. It also make the custom inputs namespaces has precedence  to top level namespace inputs.

@rafaelfranca Could you take a look?

Thank you @vala for the contributing. 
I think we can cut down the boilerplate in this method using `find`

```ruby
def attempt_mapping_with_custom_namespace(input_name)
  SimpleForm.custom_inputs_namespaces.find do |namespace|
    attempt_mapping(input_name, namespace.constantize)
  end
end
```

wdyt?