Add support for other external players

[vedgy]

Feel free to suggest players, vote for them or even implement support for some player yourself.
Player must support minimal command line interface.
Essential features of external player:

1. Directories and files for playlist can be specified as command line arguments.
2. External player notifies Venturous when playback finishes (prints something to stdout or quits).
3. External player can be shut down from command line.