LFSR additions, hors-d'oeuvres at v0.6.21

Here are three sequencers and a few toys. This is all branch v0.6.21, where there is a bit of a user guide in the README.md in that brnach.

Build it, or here are the OSX and Windows builds and visit vcvrack.com. It's on the plugin manager at vcvrack.com, too.

Short version: enjoy, explore and please help me improve these! Or if you think something similar would make a good next-my-project, I am ears.

FG8 and Psychtone LFSR note sequencers

I have added the Psychtone to this set of plugins. I have made minor changes to FG8. I have built it only for Macintosh OS X.

I had labeled it pre-release only because I expect it will be shown to be less than perfect. Cough.

LFSR w/ FG8 for Rack V0.6.0

A few changes for Rack v0.6.0, a few tweaks. I have built and tested this on OS X 10.11.6 and Rack v0.6.0.