Fixes #62

Fixes #53

Fixes #50

Fixes #41

Fixes #44

Avoids note for long argument length and allows for more consistent checking if supplied arguments are default values

Closes #33

style_* returns name of file invisibly

And add tests

Closes #28

header_background_padding is now learned from the slide padding so that left and right margins match

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.

* 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

- Also reorder code params