Fixes #3290, #2986, #3176, #1665

• Allows #RRGGBBAA colors
• Outputs color functions as-is when there are unrecognized properties (allows using CSS variables for args, for example)
• Keeps colors in the same color space when possible. e.g. spin(hsl(50, 1%, 10%), 10) becomes hsl(60, 1%, 10%) vs converting to rgba()
• Adds the rgba(<Color> [, <opacity>]) and hsla(<Color> [, <opacity>]) 1- or 2-argument forms for those functions