Feature request: ratio() classes structure option argument

[PatrickKudo]

The ratio() function assumes there is already a folder class structure (input_dir/class1, input_dir/class2, etc.). It would be helpful if there was an argument for the ratio function that allows to make this an optional assumption (e.g. splitfolders.ratio(classes=None)).

[PatrickKudo]

Just realized this was already addressed with https://github.com/jfilter/split-folders/issues/22, though I think it'd be nice to have.