feature importance: option to stack weights for categorical variable values

[arsenyinfo]

We discussed with @kmike and @lopuhin that it may be useful for analyzing datasets with categorical variables. Most likely it should be an additional wrapper function or an option for existing functions - TBD.

I'm going to work on this issue in a week.

[kmike]

If we're talking about the user-facing API, I think an option for existing functions would be better - it'd be more consistent with the rest of eil5.