Accept outside logger, do not panic

[vbmithr]

Hi, I've started using this project. I'm thinking about rewriting the parts that instantiate zap loggers and instead get them from outside, also, as a library, it should return errors instead of panicking.

What do you guys think about this?
Best,
Vincent

[ethanf]

Generally agree with this. For error handling, we want to stick as closely to the behavior of the other languages (Java/C++) to reduce surprises and make it easy to port functionality.