New feature: produce drum sounds instead of notes

[pitag-ha]

Let's add a new feature to cardio-crumble: a --drum flag, which sends the output of any of the two engines to the drum MIDI channel, i.e. to channel number 9. See #18 (comment) for more info on the "drum MIDI channel". Currently, the output is always sent to channel 0.

So, concretely the issue consists in adding a new flag to cardio-crumble, which when passed in makes that channel 9 is used instead of channel 0.

I'll tag this as medium even though it's only a few lines of code, because it means dealing with cmdliner, the library we use for command line handling.