Add support for logging to multiple locations simultaneously

[atc0005]

This is handled now via syslog + whatever output is chosen (log file, or one of stdout/stderr), but if using an io.MultiWriter the user could choose to log to ALL chosen outputs simultaneously.

Example:

package logging

import (
    "io"
    logging "log"
    "os"

    "github.com/Sirupsen/logrus"
)

var (
    log *logrus.Logger
)

func init() {
    f, err := os.OpenFile("logs/application.log", os.O_RDWR|os.O_CREATE|os.O_APPEND, 0666)

    if err != nil {
        logging.Fatalf("error opening file: %v", err)
    }

    log = logrus.New()

    //log.Formatter = &logrus.JSONFormatter{}

    log.SetReportCaller(true)

    mw := io.MultiWriter(os.Stdout, f)
    log.SetOutput(mw)
}

// Info ...
func Info(format string, v ...interface{}) {
    log.Infof(format, v...)
}

// Warn ...
func Warn(format string, v ...interface{}) {
    log.Warnf(format, v...)
}

// Error ...
func Error(format string, v ...interface{}) {
    log.Errorf(format, v...)
}

var (

    // ConfigError ...
    ConfigError = "%v type=config.error"

    // HTTPError ...
    HTTPError = "%v type=http.error"

    // HTTPWarn ...
    HTTPWarn = "%v type=http.warn"

    // HTTPInfo ...
    HTTPInfo = "%v type=http.info"
)

package main

import (

    log "logging"
)

func main() {
    log.Error(log.ConfigError, "Testing the error")
}

Reference:

https://stackoverflow.com/a/55989885/903870