Fix for issue #93 - Cannot add github account if it contains a dash

Github's own rule is:

"Username may only contain alphanumeric characters or dashes and cannot
begin with a dash."

New validator should cover both of these conditions.

The other part of this issue is, Github, as far as I can tell, does not allow non-ASCII characters. Under normal circumstances, \w will match characters of a given encoding rather than bytes. Is there a case for using \p{PerlWord} here?