Suggestion: smoothed trend in ggcoxdiagnostics()

[pbiecek]

Right now plots with residuals are enriched with horizontal line (geom_hline)
But, by adding in addition an layer with geom_smooth()
it will be easier to spot some problems with residuals (like quadratic relation).
The additional surve may be added with parameter sline (in the same spirit as for hline)

[kassambara]

fixed with the pull request #120

[kassambara]

Thanks:-)!