Add generic middleware logger interface

[beornf]

As I prefer to use a custom logger making the logger an interface means I can inject my own for request logging.

This also has the property of being fully backwards compatible with the stdlib logger.

[pkieltyka]

please show example usage

[beornf]

Overriding the default logger as below works:

middleware.DefaultLogger = middleware.RequestLogger(&middleware.DefaultLogFormatter{
    Logger: logrus.New(),
}