Background tile ratio

[martham93]

Currently background tile ratio only applies to single class classification data sets. https://github.com/developmentseed/label-maker/blob/master/label_maker/images.py#L62.
We would like to expand adding a specific ratio of background tiles to multi-class classification data sets. This addition is key for our FFDA work, and will also help other users of label-maker.

cc @wronk @drewbo @Rub21

[wronk]

All done in #159. Thanks @martham93!