Fixes #50

Fixes #41

Fixes #44

https://github.com/yihui/xaringan/pull/271

Avoids note for long argument length and allows for more consistent checking if supplied arguments are default values

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.