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 think we should strive for an implementation that works "out of the box" with `symbolicColors` while allowing you to implement your use case easily. Could you use `\InputIfFileExists` instead of `\input`? tikzDevice would create those files which you could then remove in a postprocessing, and LaTeX doesn't complain if the files are missing.