Adding 'set_feature!' to complement 'enable_feature!' and 'disable_feature!'

This allows you to write tests like

```ruby
[true, false].each do |enabled|
  context "has_my_new_feature? is #{enabled}" do
    setup do
      Arturo.set_feature!(:my_new_feature, enabled)
    end

    should "return success status" do
      assert_equal enabled, @my_obj.success?
    end
  end
end
```