New avatar option mmng (Mistery Man Next Generation (NextGen).
Related to #63
Allow to specify a new default avatar option 'mmng'. Which is very similar to the normal 'mm', but the background changes, depending on the input hash. Currently the frontend/API don't support adding more red/green/blue, but the backend function would theoretically. However, this is my first shot into that direction and I want to get the feedback first.