allow colours in workers

[botverse]

1. there are no window or document globals in workers, so useColors throws.
2. there is still an opportunity to check if we are in webkit using the userAgent

[thebigredgeek]

I am hesitant to merge something like this in without tests. We've already had a lot of issues with workers. Thoughts @TooTallNate ?

[xuset]

Hi all! I've been having the same problem with require('debug') in web workers. I tried out @botverse's fix in chrome and firefox, and everything works as expected in web workers and normal contexts. Is there anything I can help with in getting this merged in?

[thebigredgeek]

I am going to merge this, but it will be a minor semver. I will release 2.4.0 tomorrow morning with this.