Allow to skip existing titles

[timobrembeck]

At the moment, it just overwrites a file if it already exists. I think it would be cool to have a command line option to skip these songs entirely and resume with the next song in the queue.

@Bleuzen if you agree with the feature request, I would give it a try to implement this by myself. In my opinion, this is probably a pretty trivial fix, since we'd just have to use send_dbus_cmd("Next") in case the file exist, right?