More flexibility for loggers interface

[andig]

Looking at logrus it would be helpful if a Contextual's WithFields would again be Contextual. Otherwise chaining fields like e.g. module name and individual log entry fields becomes quite monotonous if in this example the module name cannot be initialized once instead of per log entry.

It would also be nice if WithFields accepted a single map-like interface instead of arrays to allow for

contextualLogger.WithFields(map[string]interface{}{
	"event": name,
	"value": payload,
}).Error("float conversion error, skipping")