* use logical `NA` that can be cast to character

* chore: Use ggplot2's default color scale `na.value`

* chore: document()

* chore: Also set `na.value = "grey50"` in `scale_xaringan_discrete()`.

* chore: Add news item

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Co-authored-by: Garrick Aden-Buie <garrick@adenbuie.com>

Co-authored-by: gadenbuie <gadenbuie@users.noreply.github.com>

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()

not working because {colorspace} doesn't force evaluation of
input arguments to scale_color_xxx_xxx() before passing down
to sequential_hcl()