Add CLICOLOR support

This implement CLICOLOR and CLICOLOR_FORCE check on terminal coloring
as defined in https://bixense.com/clicolors/

text_formatter.go

@@ -3,6 +3,7 @@ package logrus
 import (
 	"bytes"
 	"fmt"
+	"os"
 	"sort"
 	"strings"
 	"sync"
@@ -35,6 +36,9 @@ type TextFormatter struct {
 	// Force disabling colors.
 	DisableColors bool
 
+	// Override coloring based on CLICOLOR and CLICOLOR_FORCE. - https://bixense.com/clicolors/
+	OverrideColors bool
+
 	// Disable timestamp logging. useful when output is redirected to logging
 	// system that already adds timestamps.
 	DisableTimestamp bool
@@ -78,6 +82,22 @@ func (f *TextFormatter) init(entry *Entry) {
 	}
 }
 
+func (f *TextFormatter) isColored() bool {
+	isColored := f.ForceColors || f.isTerminal
+
+	if f.OverrideColors {
+		if force, ok := os.LookupEnv("CLICOLOR_FORCE"); ok && force != "0" {
+			isColored = true
+		}
+
+		if os.Getenv("CLICOLOR") == "0" {
+			isColored = false
+		}
+	}
+
+	return isColored && !f.DisableColors
+}
+
 // Format renders a single log entry
 func (f *TextFormatter) Format(entry *Entry) ([]byte, error) {
 	prefixFieldClashes(entry.Data, f.FieldMap)
@@ -100,13 +120,11 @@ func (f *TextFormatter) Format(entry *Entry) ([]byte, error) {
 
 	f.Do(func() { f.init(entry) })
 
-	isColored := (f.ForceColors || f.isTerminal) && !f.DisableColors
-
 	timestampFormat := f.TimestampFormat
 	if timestampFormat == "" {
 		timestampFormat = defaultTimestampFormat
 	}
-	if isColored {
+	if f.isColored() {
 		f.printColored(b, entry, keys, timestampFormat)
 	} else {
 		if !f.DisableTimestamp {