Add a function to force color in console output (#1724)

Add a function `ForceConsoleColor`, like `DisableConsoleColor` but to force coloring the output.

It usefull when some IDE's integrated console (like IntelliJ or Goland) are not detected as TTY, but can display colors.

Also helps if one want to output color in log file (#1590) and as a workaround for #1547.

README.md

@@ -215,9 +215,6 @@ $ go build -tags=jsoniter .
 
 ```go
 func main() {
-	// Disable Console Color
-	// gin.DisableConsoleColor()
-
 	// Creates a gin router with default middleware:
 	// logger and recovery (crash-free) middleware
 	router := gin.Default()
@@ -570,6 +567,48 @@ func main() {
 ::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" "
 ```
 
+### Controlling Log output coloring 
+
+By default, logs output on console should be colorized depending on the detected TTY.
+
+Never colorize logs: 
+
+```go
+func main() {
+    // Disable log's color
+    gin.DisableConsoleColor()
+    
+    // Creates a gin router with default middleware:
+    // logger and recovery (crash-free) middleware
+    router := gin.Default()
+    
+    router.GET("/ping", func(c *gin.Context) {
+        c.String(200, "pong")
+    })
+    
+    router.Run(":8080")
+}
+```
+
+Always colorize logs: 
+
+```go
+func main() {
+    // Force log's color
+    gin.ForceConsoleColor()
+    
+    // Creates a gin router with default middleware:
+    // logger and recovery (crash-free) middleware
+    router := gin.Default()
+    
+    router.GET("/ping", func(c *gin.Context) {
+        c.String(200, "pong")
+    })
+    
+    router.Run(":8080")
+}
+```
+
 ### 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).