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 = ""
 	}