Handling fades

[wsievern]

What's up everyone,

Over the last couple of days I have been figuring out how to implement clean (popless) fade outs when stopping playback. It's working, and sounding great!

The implementations for Android and iOS vary quite a bit because of differences between how the mSamples buffer is handled on each platform.

I'll add a PR later today @chipweinberger so you can check it out!

Will

[chipweinberger]

in my app i handle fades in the dart code

i don't think it should be handled in native code

but feel free to open a pr