Add command line option to choose sound device / file

[hackerb9]

I've sent a pull request so that modem will use the default ALSA sound device. While this is good, it is not sufficient. There should be a way for a user to specify the ALSA sound device from the command line. For example,

modem -D devicename

It would also be nice to be able to specify an audio file instead of an audio device for input. For example, I want to decode an audio file of a modem (in originate mode). I wish I could type something like, modem --audio-file-in recording.wav .