More possible feature preprocessors from scikit-feature

[rhiever]

scikit-feature provides a sklearn-like interface for even more feature preprocessing operators: https://github.com/jundongl/scikit-feature

Worth looking into. We should evaluate the feature selectors first before adding too many -- it's possible that some (or many) of them are simply not as useful as others, and we don't want to overwhelm TPOT with too many feature preprocessing operators to choose from.