Add DefaultValue text for flags

Allows a user to override the default value of a flag in the displayed help output.

Ex:
```
cli.IntFlag{
	Name: "foo, f",
	DefaultText: "random foo",
	Value: "bar",
}
```

Running `(app.name) -h` will now yield:

```
--foo value     (default: "random foo")
```

Fixes: #504

flag.go

@@ -730,7 +730,6 @@ func stringifyFlag(f Flag) string {
 	needsPlaceholder := false
 	defaultValueString := ""
 	val := fv.FieldByName("Value")
-
 	if val.IsValid() {
 		needsPlaceholder = val.Kind() != reflect.Bool
 		defaultValueString = fmt.Sprintf(" (default: %v)", val.Interface())
@@ -740,6 +739,12 @@ func stringifyFlag(f Flag) string {
 		}
 	}
 
+	helpText := fv.FieldByName("DefaultText")
+	if helpText.IsValid() && helpText.String() != "" {
+		needsPlaceholder = val.Kind() != reflect.Bool
+		defaultValueString = fmt.Sprintf(" (default: %q)", helpText.String())
+	}
+
 	if defaultValueString == " (default: )" {
 		defaultValueString = ""
 	}

flag_generated.go

@@ -16,6 +16,7 @@ type BoolFlag struct {
 	EnvVars     []string
 	Hidden      bool
 	Value       bool
+	DefaultText string
 	Destination *bool
 }
 
@@ -59,6 +60,7 @@ type DurationFlag struct {
 	EnvVars     []string
 	Hidden      bool
 	Value       time.Duration
+	DefaultText string
 	Destination *time.Duration
 }
 
@@ -102,6 +104,7 @@ type Float64Flag struct {
 	EnvVars     []string
 	Hidden      bool
 	Value       float64
+	DefaultText string
 	Destination *float64
 }
 
@@ -145,6 +148,7 @@ type GenericFlag struct {
 	EnvVars     []string
 	Hidden      bool
 	Value       Generic
+	DefaultText string
 }
 
 // String returns a readable representation of this value
@@ -187,6 +191,7 @@ type Int64Flag struct {
 	EnvVars     []string
 	Hidden      bool
 	Value       int64
+	DefaultText string
 	Destination *int64
 }
 
@@ -230,6 +235,7 @@ type IntFlag struct {
 	EnvVars     []string
 	Hidden      bool
 	Value       int
+	DefaultText string
 	Destination *int
 }
 
@@ -273,6 +279,7 @@ type IntSliceFlag struct {
 	EnvVars     []string
 	Hidden      bool
 	Value       *IntSlice
+	DefaultText string
 }
 
 // String returns a readable representation of this value
@@ -315,6 +322,7 @@ type Int64SliceFlag struct {
 	EnvVars     []string
 	Hidden      bool
 	Value       *Int64Slice
+	DefaultText string
 }
 
 // String returns a readable representation of this value
@@ -357,6 +365,7 @@ type Float64SliceFlag struct {
 	EnvVars     []string
 	Hidden      bool
 	Value       *Float64Slice
+	DefaultText string
 }
 
 // String returns a readable representation of this value
@@ -399,6 +408,7 @@ type StringFlag struct {
 	EnvVars     []string
 	Hidden      bool
 	Value       string
+	DefaultText string
 	Destination *string
 }
 
@@ -442,6 +452,7 @@ type StringSliceFlag struct {
 	EnvVars     []string
 	Hidden      bool
 	Value       *StringSlice
+	DefaultText string
 }
 
 // String returns a readable representation of this value
@@ -484,6 +495,7 @@ type Uint64Flag struct {
 	EnvVars     []string
 	Hidden      bool
 	Value       uint64
+	DefaultText string
 	Destination *uint64
 }
 
@@ -527,6 +539,7 @@ type UintFlag struct {
 	EnvVars     []string
 	Hidden      bool
 	Value       uint
+	DefaultText string
 	Destination *uint
 }
 

flag_test.go

@@ -67,6 +67,16 @@ func TestStringFlagHelpOutput(t *testing.T) {
 	}
 }
 
+func TestStringFlagDefaultText(t *testing.T) {
+	flag := &StringFlag{Name: "foo", Aliases: nil, Usage: "amount of `foo` requested", Value: "none", DefaultText: "all of it"}
+	expected := "--foo foo\tamount of foo requested (default: \"all of it\")"
+	output := flag.String()
+
+	if output != expected {
+		t.Errorf("%q does not match %q", output, expected)
+	}
+}
+
 func TestStringFlagWithEnvVarHelpOutput(t *testing.T) {
 	os.Clearenv()
 	os.Setenv("APP_FOO", "derp")
@@ -482,7 +492,6 @@ func TestFloat64FlagApply_SetsAllNames(t *testing.T) {
 	expect(t, v, float64(43.33333))
 }
 
-
 var float64SliceFlagTests = []struct {
 	name     string
 	aliases  []string
@@ -523,8 +532,6 @@ func TestFloat64SliceFlagWithEnvVarHelpOutput(t *testing.T) {
 	}
 }
 
-
-
 var genericFlagTests = []struct {
 	name     string
 	value    Generic
@@ -1100,7 +1107,6 @@ func TestParseMultiFloat64FromEnvCascade(t *testing.T) {
 	a.Run([]string{"run"})
 }
 
-
 func TestParseMultiFloat64SliceFromEnv(t *testing.T) {
 	os.Clearenv()
 	os.Setenv("APP_INTERVALS", "0.1,-10.5")
@@ -1141,7 +1147,6 @@ func TestParseMultiFloat64SliceFromEnvCascade(t *testing.T) {
 	}).Run([]string{"run"})
 }
 
-
 func TestParseMultiBool(t *testing.T) {
 	a := App{
 		Flags: []Flag{

generate-flag-types

@@ -145,6 +145,7 @@ def _write_cli_flag_types(outfile, types):
             EnvVars []string
             Hidden bool
             Value {type}
+            DefaultText string
         """.format(**typedef))
 
         if typedef['dest']: