Allow commons to be configurable

There are quite a large number of defaults in axe.commons that are difficult to update, such as:

- getRole(); are fallback roles supported
- aria.lookupTable: what roles are available, and which ones are supported
- isModalOpen(), what threshold size should this use
- getScroll(), what error bars should be used

It is currently impossible to configure these, except if options are provided at a check level. It would be helpful if commons could be configured in the axe.run options object, or as part of axe.configure.