convert `FeatureExtractor.feature_methods` from `list` to `dict`

[belevtsoff]

This will naturally allow for adding FeatureExtractor.delete_features() method which would be highly useful.

UPD: this one is not needed anymore, because another feature deletion mechanism was suggested (see closing commits)

[btel]

That is good idea, we might think of implementing it.

[belevtsoff]

I thought maybe it will be more logical or clear to name the methods hide/unhide_features instead of delete* ...

[belevtsoff]

Needs better implementation