Add -tx and -channel flags

[farmergreg]

The -tx flag causes the client to start transmitting immediately The -channel flag causes the client to join the specified channel.

[farmergreg]

I should explain the use case. A group of ham radio operators are using barnard to send audio from their radios over the internet as part of a remote control scheme using rpi's. The modifications to the code allow us to start barnard in a tmux session that joins a specific mumble channel and starts immediately transmitting whatever is coming over the radio.

[farmergreg]

wrong branch, i'll submit another pull request shortly.