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
Fine with me. Still, there might be users that set this option and then wonder why their heatmap takes ages to compile. How about restricting the number of distinct mapped colors to, say, 100 or 200 and revert to verbatim storage (with a warning) if this number is exceeded?