* The style functions are now more permissive about color names in the `color` argument. Color names should be valid CSS classes that can also be used as CSS variable names, but the `style_` function only stops with an error if the color name contains spaces. (@jdlom #69)

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.