Use a custom debug key

Allow `bows` to use a custom **on** switch.

This solves my use case where I am using the `debug` module (from __npm__) as part of @3rd-Eden's `primus` and it setting a debug key of **undefined** which is causing `bows` to log everything to the console.  

In our application this slows things down significantly for non-developers without any gain. Avoiding this collision would prevent this issue for us.

Tests included.
Do we need to move all these definitions inside the bows function? I don't think we want/need to do them on every call to bows, as that'll make everything the same color.