Improve custom logging

[mpontillo]

Closes: #621

[pycak]

I need this changes, what can I do to merge this PR?

[sio4]

Thank you for your contribution!

The main concept of this PR is a kind of "exporter". You don't want to override the whole log() function (the behavior and the format of the function) but just want to use another custom exporter for printing out the same content in the same way. Is my understanding correct?

I agree that supporting various exporters could be a great idea! However, I think this kind of extended feature should be supported by external logging libraries and this is not in the scope of Pop's simple default logger. I would like to keep this PR in mind and will consider it in the future implementation of the logger, but for now, I am going to close this PR.

One more point. I think the current behavior, printing the logs to the stdout is not a good choice so I will fix it soon.

Thanks again!

(Just left some comments on the code for future reference)