Export funcs to configure flag prefix/env hints

This will allow users to customize the prefix section or env hint
section of the flag entries in the help menu without having to
reimplement the rest of the logic required in defining FlagStringer.

flag.go

@@ -37,6 +37,14 @@ var HelpFlag Flag = BoolFlag{
 // to display a flag.
 var FlagStringer FlagStringFunc = stringifyFlag
 
+// FlagNamePrefixer converts a full flag name and its placeholder into the help
+// message flag prefix. This is used by the default FlagStringer.
+var FlagNamePrefixer FlagNamePrefixFunc = prefixedNames
+
+// FlagEnvHinter annotates flag help message with the environment variable
+// details. This is used by the default FlagStringer.
+var FlagEnvHinter FlagEnvHintFunc = withEnvHint
+
 // FlagsByName is a slice of Flag.
 type FlagsByName []Flag
 
@@ -710,13 +718,13 @@ func stringifyFlag(f Flag) string {
 
 	switch f.(type) {
 	case IntSliceFlag:
-		return withEnvHint(fv.FieldByName("EnvVar").String(),
+		return FlagEnvHinter(fv.FieldByName("EnvVar").String(),
 			stringifyIntSliceFlag(f.(IntSliceFlag)))
 	case Int64SliceFlag:
-		return withEnvHint(fv.FieldByName("EnvVar").String(),
+		return FlagEnvHinter(fv.FieldByName("EnvVar").String(),
 			stringifyInt64SliceFlag(f.(Int64SliceFlag)))
 	case StringSliceFlag:
-		return withEnvHint(fv.FieldByName("EnvVar").String(),
+		return FlagEnvHinter(fv.FieldByName("EnvVar").String(),
 			stringifyStringSliceFlag(f.(StringSliceFlag)))
 	}
 
@@ -744,8 +752,8 @@ func stringifyFlag(f Flag) string {
 
 	usageWithDefault := strings.TrimSpace(fmt.Sprintf("%s%s", usage, defaultValueString))
 
-	return withEnvHint(fv.FieldByName("EnvVar").String(),
-		fmt.Sprintf("%s\t%s", prefixedNames(fv.FieldByName("Name").String(), placeholder), usageWithDefault))
+	return FlagEnvHinter(fv.FieldByName("EnvVar").String(),
+		fmt.Sprintf("%s\t%s", FlagNamePrefixer(fv.FieldByName("Name").String(), placeholder), usageWithDefault))
 }
 
 func stringifyIntSliceFlag(f IntSliceFlag) string {
@@ -795,5 +803,5 @@ func stringifySliceFlag(usage, name string, defaultVals []string) string {
 	}
 
 	usageWithDefault := strings.TrimSpace(fmt.Sprintf("%s%s", usage, defaultVal))
-	return fmt.Sprintf("%s\t%s", prefixedNames(name, placeholder), usageWithDefault)
+	return fmt.Sprintf("%s\t%s", FlagNamePrefixer(name, placeholder), usageWithDefault)
 }

funcs.go

@@ -26,3 +26,11 @@ type OnUsageErrorFunc func(context *Context, err error, isSubcommand bool) error
 // FlagStringFunc is used by the help generation to display a flag, which is
 // expected to be a single line.
 type FlagStringFunc func(Flag) string
+
+// FlagNamePrefixFunc is used by the default FlagStringFunc to create prefix
+// text for a flag's full name.
+type FlagNamePrefixFunc func(fullName, placeholder string) string
+
+// FlagEnvHintFunc is used by the default FlagStringFunc to annotate flag help
+// with the environment variable details.
+type FlagEnvHintFunc func(envVar, str string) string