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
Yeah, I figured that, but since it was very quick to do, I just wanted to show it. I'll revert it and use a new function as you said.

To the notes: 

 1. R always resolves to gray since that is first
 1. I don't see it. But I will replace all `\definecolors` with the new function