Updates color examples and adds left-column headings demo to skeleton

Fixes #50

Fixes #41

Fixes #44

https://github.com/yihui/xaringan/pull/271

This way, warning show up in the console when rendering

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

Closes #33

E.g. primary_color and secondary_color in style_duo() become --primary and --secondary

style_* returns name of file invisibly

And add tests

Fixes #24

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