Fixes #50

Fixes #41

Fixes #44

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

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

Fixes #24

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.

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.

Also allow NULL or NA for template arguments but change default to NULL

Avoids table css being applied to help text overlay

- Also reorder code params

Add "Font" to documentation: "Text Font Size"

- write_xaringan_theme works
- mono_* works