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

- google_font() not google_fonts()
- make ranges code to avoide link

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!

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