Add always permitted parameters as a configurable option.

* This commit adds back the always_permitted_parameters configuration option to strong parameters.
* The initial pull requests where this feature was added are the following:
    - https://github.com/rails/rails/pull/12682
    - https://github.com/rails/strong_parameters/pull/174
Maybe we can do something like:

~~~ruby
# config. For instance:
#
#    config.always_permitted_parameters = %w( controller action format )
~~~

Thus we will have syntax highlighting and this reads a bit better. What do you think ? :-)