Just a very small typo I stumbled upon while reading the help. :)

* e.g. style_duo() <- duo()
* style_xaringan <- write_xaringan_theme()
* style_extra_css <- write_extra_css()
* added NEWS entry
* updated docs and /docs

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

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

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

using inst/scripts/generate_theme_functions.R

- write_xaringan_theme works
- mono_* works