Inspired by tachyons, easily add additional colors to slides using the `colors` argument. The argument takes a named list or vector and adds a CSS variable with the color name. Also added are .<color> and .bg-<name> classes.

* fix style of ggplot2, xaringan pkg refs
* fix name of theme_xaringan_restore_defaults()

Seems Droid Serif is no longer "in" Google Fonts

Need to remove all the warnings from docs