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.

* fix style of ggplot2, xaringan pkg refs
* fix name of theme_xaringan_restore_defaults()

Closes #19

not working because {colorspace} doesn't force evaluation of
input arguments to scale_color_xxx_xxx() before passing down
to sequential_hcl()

...and renamed from get_xaringanthemer_value()

Seems Droid Serif is no longer "in" Google Fonts

Need to remove all the warnings from docs

* 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 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!