Add the ability to silent/stealth login

[NuSkooler]

It would be nice to have a silent / stealth login option. This was discussed in #88 (and partially implemented)

What was implemented but not pushed (and could perhaps use a little more discussion) is this:

• Users can login with a ":" prefix to their name, e.g.: :NuSkooler
• If they have the correct ACS (defaults to GM[sysops] they are set as silent
• Silent logins don't record login history, show up in active connections, and so on.