feature: make it possible to set the global DEBUG rule in runtime

[fogine]

Currently, once a debug function is exported from the debug module eg.:

var debug = require('debug')('worker');

It's not possible to globally change enabled/disabled state of the debug function.

This'd come handy when you want to change the debug setup at runtime.
In other words, if a node process is established with disabled debug, I'd like to have an option to set the global DEBUG rule and enable some debug functions without restarting the node process.

[thebigredgeek]

@fogine we have an open PR linked above. Feel free to jump into the conversation :)