[minor] allow specific color as optional arg

When working with multiple instances of debug, it is tricky to prevent a rainbow of colors. Manually maintaining the colors by forcing a specific color should do the trick.

Per example see this issue: https://github.com/bigpipe/bigpipe/issues/37

![](https://cloud.githubusercontent.com/assets/4624/2810455/0268b54e-cdc0-11e3-8616-fc7b657e8b89.png)

Using an object hash to store the color by debug instance name will not work either, since require resolves to completely different instances.