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.

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

Closes #19

add "append" arg to `write_extra_css()`

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()

Seems Droid Serif is no longer "in" Google Fonts

Need to remove all the warnings from docs

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