add help messages to rules, updat url validation

internal/route/rules/help.go

@@ -0,0 +1,35 @@
+package rules
+
+import "strings"
+
+type Help struct {
+	command string
+	args    map[string]string // args[arg] -> description
+}
+
+/*
+Generate help string, e.g.
+
+	 rewrite <from> <to>
+		from: the path to rewrite, must start with /
+		to: the path to rewrite to, must start with /
+*/
+func (h *Help) String() string {
+	var sb strings.Builder
+	sb.WriteString(h.command)
+	sb.WriteString(" ")
+	for arg := range h.args {
+		sb.WriteRune('<')
+		sb.WriteString(arg)
+		sb.WriteString("> ")
+	}
+	sb.WriteRune('\n')
+	for arg, desc := range h.args {
+		sb.WriteRune('\t')
+		sb.WriteString(arg)
+		sb.WriteString(": ")
+		sb.WriteString(desc)
+		sb.WriteRune('\n')
+	}
+	return sb.String()
+}