ability to filter features in feature importance plots

[aikramer2]

In the event that you have many features, you may want to reduce what you see in a feature importance plot, something like "top_n" or "min_importance" or "cumulative_variance_explained".

[m-richards]

Can I work on this?

Edit: I've just submitted a pull request which implements the former of these, the latter doesn't seem useful for my applications but I could also address that easily enough.