Allow option to pass a function for debug parameter

[jankapunkt]

I am currently rebuilding our internal logging and found that when I want additional info via the "debug" flag then I can only pass a boolean to the constructor (according to the docs).

Optimally, I would want to pass a function that replaces the internal console.log as output target. The boolean flag could remain as non-breaking default.

what do you think?

[dr-dimitru]

@jankapunkt this sounds reasonable and standard in many lib

[jankapunkt]

I can go for a PR if you like, should not be too much of a task imo