New pipe commands: incr/decr volume, prev/next chapter

[ydario]

It would be nice if the pipe interface could support a increment/decrement volume instead of absolute value, so I can use the two multimedia keys (I can't read the current volume level).

And since I have also a previous and a next key, a similar pipe command will be nice to have. But I'm planning to use them to skip 120 seconds, so this is not really needed.