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

Header fonts are generally not the same font as text font, so linking the two requires tweaking other properties (padding, etc.) of the header.

Also fixed an issue with `outfile` argument not getting passed back to write_xaringan_theme!

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

using inst/scripts/generate_theme_functions.R

Allows me to generate function signature and documentation automatically, so changes to master template_variables can propagate to wrapper functions.

Also makes it easy to have all template parameters as explicit function arguments for awesome tab completion.

- write_xaringan_theme works
- mono_* works