ggplot: geom_area and geom_ribbon: fill and line colors

Summary: 
- Default fill color for `geom_area` and `geom_ribbon`: `gray20`
- Default line color for `geom_area` and `geom_ribbon`: `NA` (equivalent in Plotly to `width=0`)

geom_ribbon:
before: https://plot.ly/~pdespouy/1959
after: https://plot.ly/~pdespouy/1968

geom_area: https://plot.ly/~pdespouy/1958
after: https://plot.ly/~pdespouy/1967

Version number will be updated after revisions.

cc// @mkcor