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, because I misuse the function (this is up to debate).

I need the colorname for `fillColor` and `drawColor` and the definition of named colors. This is why I need both the name and the R string. I could live without the `colorstr`, but I would have to toss in another call to `col2name`.