Add NO_COLOR support to disable color output

This PR adds support for the enviroment variable `NO_COLOR`. If set
(regardless of its value), the `colors` package disables color output.
For more information about this environment variable please checkout
this website: https://no-color.org

closes: https://github.com/fatih/color/issues/136

README.md

@@ -127,11 +127,14 @@ fmt.Println("All text will now be bold magenta.")
  
 There might be a case where you want to explicitly disable/enable color output. the 
 `go-isatty` package will automatically disable color output for non-tty output streams 
-(for example if the output were piped directly to `less`)
+(for example if the output were piped directly to `less`).
 
-`Color` has support to disable/enable colors both globally and for single color 
-definitions. For example suppose you have a CLI app and a `--no-color` bool flag. You 
-can easily disable the color output with:
+The `color` package also disables color output if the [`NO_COLOR`](https://no-color.org) environment
+variable is set (regardless of its value).
+
+`Color` has support to disable/enable colors programatically both globally and
+for single color definitions. For example suppose you have a CLI app and a
+`--no-color` bool flag. You can easily disable the color output with:
 
 ```go
 var flagNoColor = flag.Bool("no-color", false, "Disable color output")

color.go

@@ -15,9 +15,11 @@ import (
 var (
 	// NoColor defines if the output is colorized or not. It's dynamically set to
 	// false or true based on the stdout's file descriptor referring to a terminal
-	// or not. This is a global option and affects all colors. For more control
-	// over each color block use the methods DisableColor() individually.
-	NoColor = os.Getenv("TERM") == "dumb" ||
+	// or not. It's also set to true if the NO_COLOR environment variable is
+	// set (regardless of its value). This is a global option and affects all
+	// colors. For more control over each color block use the methods
+	// DisableColor() individually.
+	NoColor = noColorExists() || os.Getenv("TERM") == "dumb" ||
 		(!isatty.IsTerminal(os.Stdout.Fd()) && !isatty.IsCygwinTerminal(os.Stdout.Fd()))
 
 	// Output defines the standard output of the print functions. By default
@@ -33,6 +35,12 @@ var (
 	colorsCacheMu sync.Mutex // protects colorsCache
 )
 
+// noColorExists returns true if the environment variable NO_COLOR exists.
+func noColorExists() bool {
+	_, exists := os.LookupEnv("NO_COLOR")
+	return exists
+}
+
 // Color defines a custom color object which is defined by SGR parameters.
 type Color struct {
 	params  []Attribute
@@ -108,7 +116,14 @@ const (
 
 // New returns a newly created color object.
 func New(value ...Attribute) *Color {
-	c := &Color{params: make([]Attribute, 0)}
+	c := &Color{
+		params: make([]Attribute, 0),
+	}
+
+	if noColorExists() {
+		c.noColor = boolPtr(true)
+	}
+
 	c.Add(value...)
 	return c
 }

color_test.go

@@ -142,9 +142,52 @@ func TestNoColor(t *testing.T) {
 
 	// global check
 	NoColor = true
-	defer func() {
+	t.Cleanup(func() {
 		NoColor = false
-	}()
+	})
+
+	for _, c := range testColors {
+		p := New(c.code)
+		p.Print(c.text)
+
+		line, _ := rb.ReadString('\n')
+		if line != c.text {
+			t.Errorf("Expecting %s, got '%s'\n", c.text, line)
+		}
+	}
+}
+
+func TestNoColor_Env(t *testing.T) {
+	rb := new(bytes.Buffer)
+	Output = rb
+
+	testColors := []struct {
+		text string
+		code Attribute
+	}{
+		{text: "black", code: FgBlack},
+		{text: "red", code: FgRed},
+		{text: "green", code: FgGreen},
+		{text: "yellow", code: FgYellow},
+		{text: "blue", code: FgBlue},
+		{text: "magent", code: FgMagenta},
+		{text: "cyan", code: FgCyan},
+		{text: "white", code: FgWhite},
+		{text: "hblack", code: FgHiBlack},
+		{text: "hred", code: FgHiRed},
+		{text: "hgreen", code: FgHiGreen},
+		{text: "hyellow", code: FgHiYellow},
+		{text: "hblue", code: FgHiBlue},
+		{text: "hmagent", code: FgHiMagenta},
+		{text: "hcyan", code: FgHiCyan},
+		{text: "hwhite", code: FgHiWhite},
+	}
+
+	os.Setenv("NO_COLOR", "")
+	t.Cleanup(func() {
+		os.Unsetenv("NO_COLOR")
+	})
+
 	for _, c := range testColors {
 		p := New(c.code)
 		p.Print(c.text)

doc.go

@@ -118,6 +118,8 @@ the color output with:
     	color.NoColor = true // disables colorized output
     }
 
+You can also disable the color by setting the NO_COLOR environment variable to any value.
+
 It also has support for single color definitions (local). You can
 disable/enable color output on the fly: