E.g. primary_color and secondary_color in style_duo() become --primary and --secondary

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.

* 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