Add transition mixin/function for Stylus

addTransition.styl

addTransition(props, duration = 300ms)
	// examples:
	//  - addTransition(color)
	//  - addTransition(color, 1s)
	//  - addTransition('color, background-color')
	//  - addTransition('color, background-color', 500ms)

	$parts = ()
	for prop in split(', ', props)
		push($parts, prop)
		will-change: unquote(join(', ', $parts))

	$parts = ()
	for prop in split(', ', props)
		push($parts, (prop + ' ' + duration + ' ease'))
		transition: unquote(join(', ', $parts))