add --no_server option to just download one cover and exit

[defaultxr]

After opening #21 I realized it probably wouldn't be that hard to add this feature myself.

As expected, this just adds a --no_server argument that will make bum exit after downloading one cover.

Additionally, in this mode, bum will exit with a status code of 1 if it fails to find a cover.

[lokesh-krishna]

bum doesn't exit even with the --no-server flag passed but neither does it detect player events.