Add option to use new user login as the display name

[MaximumRGB]

Small feature: an option to use user's login as the display name of wordpress account on registrating. (In plugin's Bouncer settings, Profile Completion block).
Reason: for Vkontakte provider WSL-plugin uses screen_name as default Display name when creating wordpress account, but very often that screen_name of vkontakte users looks like id123456789 (vk.com generates it by default), and that is a bad name to display on a website. But by enabling Allow Username change and that new option (Login as the display name on registration) users can avoid such a behavior.