Feature: Add JSON logs support

[so0k]

Currently log message format requires custom parsing to extract relevant fields in log indexing solutions:

https://github.com/abtreece/confd/blob/c27549cb0b6e21b9beb663b7d513d0203ab2f98b/pkg/log/log.go#L25

However, the used log library used does support advanced JSON logs, making the fields easily indexed sirupsen/logrus example.

If there's interest, I'd add a flag to switch on JSON logs?