Allow dot character as part of a user name

[windware-ono]

It seems your regexp to check for a valid user name in usernameRule doesn't allow for a dot, but it's a valid character for a unix user name.

^[a-z_][-a-z0-9_]{0,31}$

[tg123]

discussion #9
or it can be disable by --upstream-workingdir-allowbadusernam

[windware-ono]

No, I don't want to allow that config when you mention it as a security precaution.
Added on #9.