Extend params sanitizer, to make it easier to add/remove permitted params

- Move the default permitted parameters into ParameterSanitizer::PermittedParameters
- Add devise_permitted_parameters helper
- devise_permitted_parameters.add to add permitted parameters
- devise_permitted_parameters.remove to remove Devise's defaults
- devise_permitted_parameters.for to access the parameters for a given action
- Update 'Strong Parameters' section of README

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>
Simplify parameter sanitization proposal
Convert CHANGELOG to markdown