ggplot: geom_ribbon()

Basic implementation of geom_ribbon()

Example, running this line of code: 
gg <- ggplot(huron, aes(x=year)) + geom_ribbon(aes(ymin=level-1, ymax=level+1))
outputs: https://plot.ly/~pdespouy/1262

Particular case of NAs in data: to be implemented.

//cc @mkcor