Ports multiple octaves from python to bash and spruces up both a little.

Saw this on Reddit, loved it, and spotted a couple of places it could be even better. :)

The main change is adding the multiple octave feature to the bash script. (Why should python have all the fun?) This entailed changing the structure of the script a little bit but it should be just as readable and easier to edit further later.

I also changed the octave selection keys to something logical (z/x/c for low/middle/high, defaulting to middle). Feel free to omit this part if there's a reason you had them the way they were, but it seemed like it might have been an error because the actual keys also didn't match the instructions.

On that note, I changed the instructions slightly, both to explain the new keys and to describe the controls in terms of white keys and black keys instead of whole and half steps, which is both more accurate for people who know their music theory and clearer for people who don't. (One note alone isn't any kind of step, and the interval between e.g. E and F is a half step even though they're both white keys.)