Allow setting start and end velocity in fade in/fade out macros

[cs127]

Is your feature request related to a problem? Please describe. Fade in and Fade out already exist, but they always fade the velocity to/from 0.

Describe the enhancement you'd like I would like a feature similar to the Fade in / Fade out, but with the ability to set the velocity of the first note and the last note, and then it would generate a gradient between the two. Something like the portamento feature, but for volume instead of pitch.