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
I don't think a refactoring of `printOutput` is really necessary. Would a new function `printColorFile` with the same interface as `printOutput` also work? `TikZ_WriteColorDefinition` could gain another parameter `void (*printFun)(tikzDevDesc *, const char *, ...)`.

Two more notes:

- What about grey vs. gray?
- Below there's another code that writes `\definecolor` -- could this code make use of `TikZ_WriteColorDefinition`, too?