Add support for specifying display and/or `sndio` device to record

[morgant]

Currently, mavid will record from the X11 display specified in the DISPLAY environment variable, which one can certainly override before executing, but many X11 applications allow passing a display parameter to specify which display to record from. That would be nice to add.

Along the same lines of that, currently only the current sndio server.device audio device is recorded from. That's certainly a safe default, but it'd also be good to have an option/parameter for overriding that.

[morgant]

Naturally, issue #1 would need to be implemented first.