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 like the output of the color information to the console, but I'm also unsure if that's what we want. 

Actually, the restore of `tikzInfo->outputFile` is unnessary, as the file is already closed. I would recommend to change the signature of printOutput to be passed the console and file directly and not the ´tikzInfo` structure.