[Feature request] Log2 (octave) normalization in STFT

[Manza12]

Currently when we use the parameter 'log' for STFT frequency scale it spaces logarithmically the frequencies with base e. I think it will be nice to have also logarithmic spacing with base 2, such that it will be more music related.
It may be just a new freq_scale option, like 'log2'. If you want I can do a PR :)

[KinWaiCheuk]

Yes, a log2 scale would be useful in some cases! Sure, please go ahead!