Add always_permitted_parameters as an option.

* This commit adds back the always_permitted_parameters
  configuration option to strong paramaters.
* 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
Improvements per code review.

* General style fixes.
* Add changes to configuration guide.
* Add missing tests.
Merge pull request #15933 from rafael/master

Add always permitted parameters as a configurable option.

[Rafael Mendonça França + Gary S. Weaver]