allow more complex color definitions

[HelloLudger]

This does two things:

1. add "color" to classes and
2. allow mixed colors, as plantuml does:

class singlecolor #FFAA00 {}
class multicolor1 #FFAA00-0000FF {}
class multicolor2 #FFAA00|0000FF {}
class multicolor3 #FFAA00/0000FF {}
class multicolor4 #FFAA00\0000FF {}