Option for 'clean' usernames (tomanthony rather than Tom1.Anthony) from email address.

Just a simple configurable feature so that usernames generated from email address can optionally be a bit neater. That is they will be forced to lower case and all non-alpha characters stripped (including numbers). This creates slightly neater usernames.

There is a chance that usernames can clash, but that is no different to the current situation.

Thanks for all the work on this great library, btw! :)