[Feature Request] volumeup and volumedown commands

Please, implement `volumeup` and `volumedown` commands.
Or it could be just change `set_volume` command to accept `+/–NN` values.

Currently, I have to do `volumeup` like that:
```shell
clementine-remote set_volume  `clementine-remote status | awk '$4 ~/volume=/ { gsub(/[a-z=,]/,""); print $4+4 }'`
```