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
No, it is linear. In fact, this is done exactly the same way that R does it (the function you referenced [above] (https://github.com/wch/r-source/blob/fbf5cdf29d923395b537a9893f46af1aa75e38f3/src/library/grDevices/src/colors.c#L1399) ). For the sake of simplicity, I picked the simple solution, although it could be easily sorted and searched in logarithmic time.