New feature: plots for competing risks

[pbiecek]

Plots that handle multiple stat models:

• based on geom_area (as suggested by @teigentler)
• ggplot that replicates plot from cuminc{cmprsk} and handles grouping variables

Suggested name: ggcompetingrisks (fell free to prose something else in comments).

[kassambara]

ggcompetingrisks() sounds good for me.

[pbiecek]

Submitted as #169. CIC are implemented for cuminc objects and survfitms (as suggested by @teigentler)

Both types of plots support grouping variable as a separate facets.

[kassambara]

Thank you for this outstanding work!!

[teigentler]

Perfect. Thank you!