Add support for choosing a virtual server by voice port

If a teamspeak server has multiple virtual servers running, all commands must be sent after defining which server to use. The ServerQuery command for this is "use" with either a serverid or a server port. By default, users cannot list the virtual servers a Teamspeak server has, so the best way around this is to define the Teamspeak server by its voice port, e.g. "use port=[voice port]."

This allows server owners to define HUBOT_TEAMSPEAK_VOICE_PORT, which will, if defined, be used to select that server, allowing all subsequent commands to work. Without this, servers that have multiple virtual servers cause the entire hubot script to fail, as the server will respond with "invalid serverid" to all commands run.