Make caller formatting configurable

[skipor]

I have problem like #304, but I want truncate file paths to <parent_folder_name/file_name>. Because of really long my app package path, even truncating to $GOPATH is not enough.
I know that in #201 decided that

Support for mutative external hooks has been deemed "not supported", and won't be coming back.

And, as result, there is no way no modify Entry.Caller via Option
So I found only way to do this: wrap zapcore.Encoder, and truncate Entry.Caller.File before calling wrapped Encoder. But that is really ugly way, and can't be used with beautiful Config helpers.

My best idea how to improve usability in this case is add kind of EncodeCaller into EncoderConfig and use it to encode Caller. Also, this is nice way to solve #304.

Other ways to improve usability:

• Add modifying hooks, that are called after Core check
  • Not best idea.
• Allow wrap Encoder easier.
  • Make Config methods exported to add ability for manual Logger build.

[akshayjshah]

Good point - I hadn't thought about making the caller formatting configurable, but it's a good idea. Since you have a PR open, I'll move discussion there.

[akshayjshah]

Fixed in #327.