Cleaner StrongParameters whitelisting

Ever since SP replaced attr_protected / accessible, I felt a little bit uncomfortable with the additional method it requires you to add to every controller. Since almost all the additional *_params look the same (require + permit something) I created this helper to make it easier to whitelist parameters list for a specific action:

so instead of doing something like this - 
```ruby
  class UsersController < ActionController


    def new
      @user = User.new
    end

    def create

      # Here we are using the parameter whitelisting method
      @user = User.new(user_params)
      if @user.save
        redirect_to users_url, notice: "User created"
      else
        render :new
      end
    end


    private

    # Parameter whitelisting method
    def user_params
      params.require(:user).permit(:name, :age)
    end
  end
```

this change allows you use the `whitelist_params` method:

```ruby
  class UsersController < ApplicationController

    filter_parameters create: {user: [:name, :age]}

    def create
      @user = User.new(create_params) # => create_params is available via BetterStrongParams and the filter_parameters DSL.
      if @user.save
        redirect_to treasure_url
      else
        redirect_to jail_url
      end
    end
  end
```

Two things:

1. Not sure I added the method in the right place, under ActionController::Base
2. Tests. feels like I could use a few more ideas.