Compatibility with future ggplot2

[teunbrand]

Hi Garrick,

In ggplot2, we've made updates to update_geom_defaults() (tidyverse/ggplot2#5781) that turned out to be incompatible with {xaringanthemer}.
The issue stems from theme_xaringan_restore_defaults() where the old_ggplot_defaults cache became invalid due to having both spellings of colour and color.
This is prevented by using colour as ggplot2 uses internally.

Feel free to test out with the development branch of ggplot2:

pak::pak("tidyverse/ggplot2")

Our plans for the next release of ggplot2 are still very far away, so there is no hurry in this.

[gadenbuie]

Thanks @teunbrand!