Add an argument to determine which sound device to use for audio

[redargon]

It would be beneficial to be able to specify which sound output device that omxplayer should use. In some instances I would like to use the omxplayer -o alsa option, for example, so that audio can be routed to the alsa driver which is configured to play audio as required by the hardware. The default option for omxplayer is that it plays audio through the hdmi port, which doesn't help if the user has headphones (or another sound system) connected to the pi.