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
To be precise: multiple `%s` is a problem, none is not. There are still some issues with `onefile = FALSE` that need to be solved. The current code puts all the definitions in the `filename_nnn_color.tex` but inputs `filename_001_color.tex` in `filename_001.tex` and so on.