No Special Characters in Usernames

This is a fairly useless plugin.  It just disallows usernames which have characters other than a-z, 0-9 and _ in them (can allow more characters by changing the setting under "User Registration and Profile Options").
I recall someone requesting this, which is why I made it.

Otherwise, just posting it here to increase my post count.

[ZiNgA BuRgA]

Done

[RateU]

Thank you very much, Yumi!