ggroc on smooth.roc objects

[arubio2]

It would be great if ggroc could work also on smooth.roc objects.
By the way, good and nice work.

[xrobin]

This should be doable rather easily.

[xrobin]

This feature is now available for testing on the master branch.

[xrobin]

The feature has been merged into pROC 1.17.0 which is ready for release on CRAN (see #89).