And add tests

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

- Update license, description
- Add documentation, images and gifs

- write_xaringan_theme works
- mono_* works