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