Add time stretching / pitch shifting

[xavierfav]

As cited in #8, it would be nice to time stretch and/or pitch shift sounds from Freesound.
This could allow when playing with loops, to synchronize their bpm and thus expand the possibilities of combination.
Moreover, it could allow to have a sampler that could take different pitch values as input, and easily create a melody.

The base algorithm for this is the well known phase vocoder with based on STFT analysis and overlap & add for synthesis. However there is no agreed or well defined available implementation for web browser [Dias et al. 2016].

Some implementations:

• VexWarp (time stretching & pitch shifting - not real time)
• PhaseVocoder.js (time stretching - real time)
• Pitch shifter (pitch shifting - real time)