Add 43tet and 50tet tunings

[cgibbard]

I quickly added support for a couple additional meantone tunings that people might like to have available, since there were a couple buttons to spare in the tuning row. 43tet makes maj7 chords really pretty, since its E-F semitone is only 0.1 cents off from 16/15, and it's probably the more important of the two here.

As for why 50, well, I'm not sure, it's just another point on the meantone line that I've been rather fond of lately, but if someone wants to champion something different I'd be happy to give up the spot. :)

If you have some other idea for future use of these buttons, no worries, in any case, it was quite an easy change to make. Thanks very much for making the firmware open source!

I'll also take the opportunity to say since I got my Striso this afternoon, I've been having a lot of fun with it! It'll be the perfect portable complement to my Lumatone, just need to work out the portable speakers/power to be able to take it on walks. :)