[Feature request] ggtext should be coord_polar() aware

[lucazav]

Graphing in polar coordinates with ggplot2 is really very simple.
The only problem is that the labels are not affected by the coord_polar() function. As Hadley himself commented in this post, it would be handy for an add-on to take care of this feature.

I was thinking it would be really useful to provide a parameter in the geom_richtext() function that would enable the text to curve according to the polar coordinates enabled by coord_polar().