No check for multiple instances

[llyram]

There is no check against multiple instances running which in my specific use case is undesirable.
I use a tiling window manager (awesomeWM) and autostart volctl using awesomewm config. So if I refresh the window manager then another instance of volctl starts in the systray which does not happen with the other systray apps that I use.
A simple fix is to check for another instance running and terminate.

[buzz]

Duplicate of #57