Add reverb effect after master gain

[jsakas]

https://developer.mozilla.org/en-US/docs/Web/API/ConvolverNode

I think it would be awesome if we added a reverb effect after the master gain, this would really bring some of the sounds we are creating to life. It seems like convolution is a somewhat advanced topic, so I think for a basic implementation we could implement a 2-knob reverb effect that would be Dry/Wet and Decay

[SkylerWilliams]

🙌 global effects! maybe a compressor could be in the works too :P

[jsakas]

I would love it! See: #29

[jsakas]

Also should note that a reverb or compressor effect does not need any envelope or per-note modification, so it can be created as a new class in Subtractor.constructor() and attached really easily via the existing UI components.