Improved contrast with CSS variable support

• Plugin: Color
  • NEW: var-contrast() and var-contrasted()
    provide CSS variable parallels
    to contrast() and contrasted()
  • NEW: User-defined contrast defaults
    (contrast-light and contrast-dark)
    can be defined with or without the _ private-color prefix
  • NEW: Contrast-related functions and mixins
    provide a smart second light-or-dark default option
    when only one is defined (previously an error)