[RFC] child debug instances

[thebigredgeek]

discuss!

const log1 = debug('foo');
log1.use((data, next) => ... ); //apply some new middleware
const log2 = log.child('bar');
// uses middleware of ancestors unless overridden on this instance
log2('hello world'); // => foo:bar hello world

[DiegoRBaquero]

This is a really nice idea