Add CustomRecovery builtin middleware (#2322)

* Add CustomRecovery and CustomRecoveryWithWriter methods

* add CustomRecovery example to README

* add test for CustomRecovery

* support RecoveryWithWriter(io.Writer, ...RecoveryFunc)

README.md

@@ -496,6 +496,39 @@ func main() {
 }
 ```
 
+### Custom Recovery behavior
+```go
+func main() {
+	// Creates a router without any middleware by default
+	r := gin.New()
+
+	// Global middleware
+	// Logger middleware will write the logs to gin.DefaultWriter even if you set with GIN_MODE=release.
+	// By default gin.DefaultWriter = os.Stdout
+	r.Use(gin.Logger())
+
+	// Recovery middleware recovers from any panics and writes a 500 if there was one.
+	r.Use(gin.CustomRecovery(func(c *gin.Context, recovered interface{}) {
+		if err, ok := recovered.(string); ok {
+			c.String(http.StatusInternalServerError, fmt.Sprintf("error: %s", err))
+		}
+		c.AbortWithStatus(http.StatusInternalServerError)
+	}))
+
+	r.GET("/panic", func(c *gin.Context) {
+		// panic with a string -- the custom middleware could save this to a database or report it to the user
+		panic("foo")
+	})
+
+	r.GET("/", func(c *gin.Context) {
+		c.String(http.StatusOK, "ohai")
+	})
+
+	// Listen and serve on 0.0.0.0:8080
+	r.Run(":8080")
+}
+```
+
 ### How to write log file
 ```go
 func main() {