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

...and renamed from get_xaringanthemer_value()

* e.g. style_duo() <- duo()
* style_xaringan <- write_xaringan_theme()
* style_extra_css <- write_extra_css()
* added NEWS entry
* updated docs and /docs

Also fixed an issue with `outfile` argument not getting passed back to write_xaringan_theme!

using inst/scripts/generate_theme_functions.R

- write_xaringan_theme works
- mono_* works