Feature request: deltas-deltas in feature_extraction()

[bagustris]

Again, thanks for your hard and beautiful works!

I would like to request a feature for feature_extraction() function. Given deltas are extracted by default (68 feature in total), it is reasonable to add the deltas-deltas feature. I think it doesn't require lot of works. Also, this kind of feature is common in speech processing, e.g., deltas-deltas of MFCC. I would like to request both deltas and deltas-deltas are not enabled by default (deltas=False).