Make logger require an interface instead of a concrete implementation

[mcuelenaere]

It is much easier to integrate 3rd-party log implementations by requiring an interface instead of requiring a Logger struct (which implements this interface).

[lance6716]

please fix lint errors, rest lgtm

And is the Contextual from https://github.com/siddontang/go-log/blob/master/loggers/loggers.go better suit your use case? Seems it's more flexible.

[mcuelenaere]

And is the Contextual from https://github.com/siddontang/go-log/blob/master/loggers/loggers.go better suit your use case? Seems it's more flexible.

It is more flexible, but AFAIK it is not implemented by the logger, so it would break things: https://github.com/siddontang/go-log/blob/master/log/logger.go#L63

[mcuelenaere]

please fix lint errors, rest lgtm

Should be fixed now.