request: add actual adsr envelope

[M0ahgra]

beepbox ALMOST has a full adsr envelope, y not go all the way? thats basically the entire request.

[Mid-The-Modder]

For the part about attack coming before the note starts, it is very difficult to predict upcoming notes and compute their "fades" early on like such shown here, so no high hopes on that.

For everything else though, generally custom envelopes would be pretty nifty. Maybe something for LFO/incremented envelopes too if this is ever considered?