And add tests
Fixes #24
Closes #28
header_background_padding is now learned from the slide padding so that left and right margins match
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.