Add default arguments for shell commands feature

[eugercek]

Both default audio players(aplay and afplay) don't have a configuration file.
Generally minimal audio players such as ffplay, mpg123 don't have a configuration file either.
So if a user wants a default option they need to compile their program or set 7 sound options with the same string.

Default arguments are useful for volume and no window settings. For example, I use:

(setq org-pomodoro-audio-player "/usr/bin/ffplay")
(setq org-pomodoro-default-args "-volume 70 -autoexit -nodisp")))

By default aplay plays sound at the current audio level. So one can get an ear blast pretty easily.