add classes_to_plot option to plot_cumulative_gain

[etabeta78]

I think it could be useful, when one wants to plot only e.g. class 1, to have an option to produce consistent plots for both plot_cumulative_gain and plot_roc

At the moment, instead, only plot_roc supports such option.

Thanks a lot

[reiinakano]

I guess this makes sense. Open to PRs

[lugq1990]

@reiinakano I have changed plot_cumulative_gain for choosing classes for ploting also with 'micro' and 'macro' as default ploting curve. You can check it out.

[lugq1990]

Because pytest of unit test for plot_cumulative_plot is not allowed for multi-classes plot originally, so this may cause pytest can't pass.