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
Thanks for looking into those issues. I'm not sure about defining *all* XXXXXX colors -- what if you plot a heat map with 10000 tiles?

The file name should be configurable with a sensible default. You could allow the user to specify a `sprintf` pattern, just like with the `file` argument of e.g. the `pdf()` function. How about `"%s_colors"` as default?