xaringanthemer 0.3.0

Renamed all xaringanthemer functions that create CSS styles to use the prefix “style_”. For example, mono_light() is now style_mono_light(). Additionally, write_xaringan_theme() is now style_xaringan() and write_extra_css() is style_extra_css(). Overall, this change makes the xarignanthemer API much cleaner and easier to navigate. Previous function names are deprecated but still work.
Added background_header_auto parameter that adds a background under the first h1 element (i.e. # Slide Title) on a slide. When enabled, the background is added automatically to slides, as long as they aren’t inverse, center, middle, or bottom-styled slides. When disabled, you can still enable slide title headers by adding the header_background class to a slide. (thanks, @Btibert3, #10)
The underlying template now uses the whisker package.