New port: audio/amsynth

Amsynth is an analog modelling (a.k.a virtual analog) software
synthesizer.

It mimics the operation of early analog subtractive synthesizers with
classic oscillator waveforms, envelopes, filter, modulation and
effects.

WWW: http://amsynth.github.io/

PR: 223361
Submitted by: owen94012@gmail.com