ggplot: colours: geom_polygon() defaults, and transparency

Summary:
- When using `colour="NA"`, change `toRGB()` output to `transparent` (compatible with Plotly)
- Default `colour` for geom_polygon(): `NA`
- Default `fill` for geom_polygon(): `grey20`
- Adapt geom_ribbon() and geom_area() to new `toRGB()`
- Improve tests

ggplot2 reference: https://github.com/hadley/ggplot2/blob/4bb9270ef4d5d5062353438fd99d17b6f6de98a2/R/geom-polygon.r#L91

geom_polygon() default `line` colour:
Before: https://plot.ly/~pdespouy/2061
After (with `colour="NA"`): https://plot.ly/~pdespouy/2062

geom_polygon() default `fill` colour:
Before: https://plot.ly/~pdespouy/2077
After (with `fill="grey20"`): https://plot.ly/~pdespouy/2072

Image diffs can be seen here: 2921585, and here: d23c12e

Version number will be updated after revisions.

/cc @mkcor