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

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