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
Strange. I can't seem to [find](https://github.com/wch/r-source/search?utf8=%E2%9C%93&q=col2name&type=Code) it in R-devel. However, if I'm not mistaken, colors for which no name is defîned are converted to `#XXXXXX` or `#XXXXXXXX` -- will `\definecolor` parse this?