Add SimpleForm.custom_inputs_namespaces config

This allows to namespace custom inputs classes to avoid conflicts with other form libraries.

This fixes #1077
@rafaelfranca I thought about it but it seemed easier to separate "custom" namespaces from default one, so end user don't override them inadvertently by setting `SimpleForm.custom_inputs_namespaces = ['Module']`. But we can use `mattr_reader` instead of `mattr_accessor` to declare the config, so this can't happen. 

Another thing is the priority order of overrides : I thought we should let top-level overrides have the highest priority, then our custom namespaces, and last the default SimpleForm inputs. But if we set by default `@@custom_inputs_namespaces = ['Object', 'SimpleForm::Inputs', 'SimpleForm::FormBuilder']` the custom namespaces added with `<< 'MyNamespace'` will never override the default inputs.
One solution would be to set the default order to : `['SimpleForm::Inputs', 'SimpleForm::FormBuilder', 'Object']` and process them in reverse order. But wouldn't it seem a bit counter-intuitive ? 

Tell me what you think about it !