add option to ignore default input class

The new `input_class` global configuration is really helpful for a Bootstrap 3 app, but gets in the way with checkboxes because they do not like the `form-control` class. See http://getbootstrap.com/css/#forms for an example.

This PR adds a new option to inputs that ignores the default input class.

`= f.input :example_boolean, ignore_default_input_class: true, label: false, inline_label: "Check me?"`