Added missing vendored packages

vendor/github.com/Luzifer/go_helpers/str/slice.go

@@ -0,0 +1,21 @@
+package str
+
+// AppendIfMissing adds a string to a slice when it's not present yet
+func AppendIfMissing(slice []string, s string) []string {
+	for _, e := range slice {
+		if e == s {
+			return slice
+		}
+	}
+	return append(slice, s)
+}
+
+// StringInSlice checks for the existence of a string in the slice
+func StringInSlice(a string, list []string) bool {
+	for _, b := range list {
+		if b == a {
+			return true
+		}
+	}
+	return false
+}