Add support for pitch shifting in AudioSource

[jgranick]

From @matrefeytontias on November 19, 2016 3:37

Before the major refactor of version 4 and upwards (that is, at least in 3.6.1), openfl.media.SoundChannel exposed an undocumented pitch member variable that allowed for pitch shifting on native targets. Looking through the documentation, there doesn't seem to be a way to do that anymore, which is too bad since this is a very good thing to have in a game.

Copied from original issue: openfl/openfl#1350

[jgranick]

From @alexandervrs on December 1, 2016 9:6

Yes, I agree that'd be a great thing to have for game audio

[jgranick]

Ultimately, this would be an improvement to the Lime AudioSource API. We aren't going to change the API on the OpenFL side 😄