[Plugin] Support Kerberos proxy authentication

[MultiMote]

Organizations often use Windows domain controllers and SSO authentication. This includes the frequent use of forward proxy servers to control user Internet activity.

I created a simple plugin with Kerberos auth support: https://github.com/MultiMote/proxy.py-kerberos

Due to my unfamiliarity with the Kerberos/GSS protocol, I can't be sure that everything works as expected.

Maybe related to #1091

Link to Squid proxy documentation (may be useful): https://wiki.squid-cache.org/ConfigExamples/Authenticate/Kerberos