Logger can be overridden from external sources

[Ruxton]

Allows logger to be overriden with an external logger.

  flags :=  log.Ldate | log.Ltime | log.LUTC
  logger := log.New(os.Stdout, "[stuff] ", flags)

  ipFilter,err := ipfilter.New(ipfilter.Options{
    AllowedIPs: configuration.IPWhiteList,
    BlockByDefault: true,
    Logger: logger,
  })

[jpillora]

Thanks for the PR, though minor change:

    Logger interface {
        Fatal(v ...interface{})
        Fatalf(format string, v ...interface{})
        Fatalln(v ...interface{})
        Flags() int
        Output(calldepth int, s string) error
        Panic(v ...interface{})
        Panicf(format string, v ...interface{})
        Panicln(v ...interface{})
        Prefix() string
        Print(v ...interface{})
        Printf(format string, v ...interface{})
        Println(v ...interface{})
        SetFlags(flag int)
        SetOutput(w io.Writer)
        SetPrefix(prefix string)
    }

should be

    Logger interface {
        Printf(format string, v ...interface{})
    }

since this package only makes use of Printf

[Ruxton]

@jpillora have fixed it up

[jpillora]

Merged. Thanks!