Add flag for symbolic color names

This is a rough implementation for issue https://github.com/yihui/tikzDevice/issues/69. I'm waiting for feedback
Hehe, funny, now that I look at it. The way I refactored it, this is not possible anymore, since I misuse the function (as stated above). I want to have only one function to define colors and the output depends on where it has been called (from `TikZ_WriteColorDefinitions` or from `TikZ_DefineDrawColor`).

Now, I would refactor printOutput, what do you say?