Mean ROC curve in ggroc()

[AngCamp]

It would be very nice if the package contained a means of producing a mean ROC curve from a list of roc objects. This stack post illustrates the desired output. Sklearn in python can do this quite easily as well, as well as plotting the S.D. along the curve as well.

https://stackoverflow.com/questions/52467915/plotting-mean-roc-curve-for-multiple-roc-curves-r

[xrobin]

This is the same as the request in #41, except specifically requesting support in the ggroc function.