Support serving on unix domain socket

Unix sockets provide a nice alternative to TCP sockets that provide more security on systems on which multiple users have shell access. A service running on a TCP socket can be curl'd by anyone on the system, whereas a unix socket can be secured with ordinary file permissions.

It looks like some rack servers such as thin already support listening on unix sockets. It would be nice to support this, which would mean:

• optionally using a different server from the default webrick
• providing a convenient cli switch, or rewording the help text for existing ones