String and Int slices called twice should append not overwrite

This allows users to do things like
```
cmd --filename=file1 --filename=file2 --filename=file3,file4
```
And internally we will get
```
[]string{"file1", "file2", "file3", "file4"}
```

1 files changed, 23 insertions, 3 deletions

diff --git a/string_slice_test.go b/string_slice_test.go
index c37a91a..d5c0d03 100644
--- a/string_slice_test.go
+++ b/string_slice_test.go
@@ -28,17 +28,37 @@ func TestSS(t *testing.T) {
 	}
 	for i, v := range ss {
 		if vals[i] != v {
-			t.Fatal("expected ss[%d] to be %s but got: %s", i, vals[i], v)
+			t.Fatalf("expected ss[%d] to be %s but got: %s", i, vals[i], v)
 		}
 	}
 
 	getSS, err := f.GetStringSlice("ss")
 	if err != nil {
-		t.Fatal("got an error from GetStringSlice(): %v", err)
+		t.Fatal("got an error from GetStringSlice():", err)
 	}
 	for i, v := range getSS {
 		if vals[i] != v {
-			t.Fatal("expected ss[%d] to be %s from GetStringSlice but got: %s", i, vals[i], v)
+			t.Fatalf("expected ss[%d] to be %s from GetStringSlice but got: %s", i, vals[i], v)
+		}
+	}
+}
+
+func TestSSCalledTwice(t *testing.T) {
+	var ss []string
+	f := setUpSSFlagSet(&ss)
+
+	in := []string{"one,two", "three"}
+	expected := []string{"one", "two", "three"}
+	argfmt := "--ss=%s"
+	arg1 := fmt.Sprintf(argfmt, in[0])
+	arg2 := fmt.Sprintf(argfmt, in[1])
+	err := f.Parse([]string{arg1, arg2})
+	if err != nil {
+		t.Fatal("expected no error; got", err)
+	}
+	for i, v := range ss {
+		if expected[i] != v {
+			t.Fatalf("expected ss[%d] to be %s but got: %s", i, expected[i], v)
 		}
 	}
 }