Add ominus symbol to symbols.js, move psi next to Psi for code legibility

I did not change any functional code whatsoever.  I just added support for the ominus symbol(u2296) to complement the oplus symbol that had already been added.

I also had some trouble finding psi, so I moved it up right next to Psi for maximum legibility.  It's a small change, but I hope it helps!
Happy to move it back.  It was originally just clearer to me because Psi is u03c7 and psi is u03c8.  Didn't mean to mess with the order!