header_background_padding is now learned from the slide padding so that left and right margins match

Now, font size is inherited from `base_font_size` which should be in absolute units.

See google/fonts issue number 1223

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.

Closes #19

* 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!

- write_xaringan_theme works
- mono_* works