[Feature Request] Implement playback speed

[mnadareski]

Default playback speed is 100%, allow for changing this value to something else, possibly in a range like 25% to 200%. Bonus effort if multiple ways of playing back at different speeds are implemented (stretching vs. pitch, etc.)

[mnadareski]

NWaves, the library we're already using to deal with emphasis filtering, has the ability to change playback rate by stretching samples. Ensure that this doesn't change the length of the sample.