Add Ability To De-Register Module Scenarios

If a module that has scenarios registered is disabled/uninstalled and removed from the file system the scenarios that behatrunner has registered are still registered which causes an error when behatrunner tries to run the tests. There are likely other use cases where it would be desirable to de-register scenarios as well. The attached patch adds a new drush command that will de-register a module's scenarios or all scenarios if a module is not specified.

The patch has not been completely tested but it did work for the simple use case I have right now. I'll do more testing but I'd like to get feedback regarding the patch - is this something that should be included and if so what changes/improvements do I need to make?