Add LoggerWithFormatter method (#1677)

* Add LoggerWithFormatter

* Add tests for LoggerWithFormatter & LoggerWithConfig

* Add note for README

* Add tests for DefaultLogFormatter

* Add comment

* Change DefaultLogFormatter to a private method

README.md

@@ -35,6 +35,7 @@ Gin is a web framework written in Go (Golang). It features a martini-like API wi
     - [Blank Gin without middleware by default](#blank-gin-without-middleware-by-default)
     - [Using middleware](#using-middleware)
     - [How to write log file](#how-to-write-log-file)
+    - [Custom Log Format](#custom-log-format)
     - [Model binding and validation](#model-binding-and-validation)
     - [Custom Validators](#custom-validators)
     - [Only Bind Query String](#only-bind-query-string)
@@ -528,6 +529,43 @@ func main() {
 }
 ```
 
+### Custom Log Format
+```go
+func main() {
+	router := gin.New()
+
+	// LoggerWithFormatter middleware will write the logs to gin.DefaultWriter
+	// By default gin.DefaultWriter = os.Stdout
+	router.Use(gin.LoggerWithFormatter(func(param gin.LogFormatterParams) string {
+
+		// your custom format
+		return fmt.Sprintf("%s - [%s] \"%s %s %s %d %s \"%s\" %s\"\n",
+				param.ClientIP,
+				param.TimeStamp.Format(time.RFC1123),
+				param.Method,
+				param.Path,
+				param.Request.Proto,
+				param.StatusCode,
+				param.Latency,
+				param.Request.UserAgent(),
+				param.ErrorMessage,
+		)
+	}))
+	router.Use(gin.Recovery())
+
+	router.GET("/ping", func(c *gin.Context) {
+		c.String(200, "pong")
+	})
+
+	router.Run(":8080")
+}
+```
+
+**Sample Output**
+```
+::1 - [Fri, 07 Dec 2018 17:04:38 JST] "GET /ping HTTP/1.1 200 122.767µs "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.80 Safari/537.36" "
+```
+
 ### Model binding and validation
 
 To bind a request body into a type, use model binding. We currently support binding of JSON, XML, YAML and standard form values (foo=bar&boo=baz).