Store the length of Args when a -- is found in commandline

This allows a program to know what args came before the -- and what args
came after.

flag.go

diff --git a/flag.go b/flag.go
index 66ef096..57d9339 100644
--- a/flag.go
+++ b/flag.go
@@ -140,6 +140,7 @@
 	formal            map[NormalizedName]*Flag
 	shorthands        map[byte]*Flag
 	args              []string // arguments after flags
+	argsLenAtDash     int      // len(args) when a '--' was located when parsing, or -1 if no --
 	exitOnError       bool     // does the program exit if there's an error?
 	errorHandling     ErrorHandling
 	output            io.Writer // nil means stderr; use out() accessor
@@ -292,6 +293,13 @@
 	return result, nil
 }
 
+// ArgsLenAtDash will return the length of f.Args at the moment when a -- was
+// found during arg parsing. This allows your program to know which args were
+// before the -- and which came after.
+func (f *FlagSet) ArgsLenAtDash() int {
+	return f.argsLenAtDash
+}
+
 // MarkDeprecated indicated that a flag is deprecated in your program. It will
 // continue to function but will not show up in help or usage messages. Using
 // this flag will also print the given usageMessage.
@@ -740,6 +748,7 @@
 
 		if s[1] == '-' {
 			if len(s) == 2 { // "--" terminates the flags
+				f.argsLenAtDash = len(f.args)
 				f.args = append(f.args, args...)
 				break
 			}
@@ -806,6 +815,7 @@
 	f := &FlagSet{
 		name:          name,
 		errorHandling: errorHandling,
+		argsLenAtDash: -1,
 		interspersed:  true,
 	}
 	return f
@@ -822,4 +832,5 @@
 func (f *FlagSet) Init(name string, errorHandling ErrorHandling) {
 	f.name = name
 	f.errorHandling = errorHandling
+	f.argsLenAtDash = -1
 }

flag_test.go

diff --git a/flag_test.go b/flag_test.go
index 1b4005c..e17b2aa 100644
--- a/flag_test.go
+++ b/flag_test.go
@@ -387,6 +387,9 @@
 	} else if f.Args()[1] != notaflag {
 		t.Errorf("expected argument %q got %q", notaflag, f.Args()[1])
 	}
+	if f.ArgsLenAtDash() != 1 {
+		t.Errorf("expected argsLenAtDash %d got %d", f.ArgsLenAtDash(), 1)
+	}
 }
 
 func TestParse(t *testing.T) {
@@ -424,6 +427,9 @@
 	if f.Changed("invalid") {
 		t.Errorf("--invalid was changed!")
 	}
+	if f.ArgsLenAtDash() != -1 {
+		t.Errorf("Expected argsLenAtDash: %d but got %d", -1, f.ArgsLenAtDash())
+	}
 }
 
 func replaceSeparators(name string, from []string, to string) string {
@@ -713,6 +719,9 @@
 	if f.Args()[1] != arg2 {
 		t.Errorf("expected argument %q got %q", arg2, f.Args()[1])
 	}
+	if f.ArgsLenAtDash() != 0 {
+		t.Errorf("expected argsLenAtDash %d got %d", 0, f.ArgsLenAtDash())
+	}
 }
 
 func TestDeprecatedFlagInDocs(t *testing.T) {