caddyhttp: Implement caddy respond command (#4870)

cmd/main.go

@@ -338,6 +338,7 @@ func flagHelp(fs *flag.FlagSet) string {
 
 	buf := new(bytes.Buffer)
 	fs.SetOutput(buf)
+	buf.Write([]byte("(NOTE: use -- instead of - for flags)\n\n"))
 	fs.PrintDefaults()
 	return buf.String()
 }
@@ -480,3 +481,16 @@ func CaddyVersion() string {
 	}
 	return ver
 }
+
+// StringSlice is a flag.Value that enables repeated use of a string flag.
+type StringSlice []string
+
+func (ss StringSlice) String() string { return "[" + strings.Join(ss, ", ") + "]" }
+
+func (ss *StringSlice) Set(value string) error {
+	*ss = append(*ss, value)
+	return nil
+}
+
+// Interface guard
+var _ flag.Value = (*StringSlice)(nil)