command: implement help builder

Signed-off-by: Ophestra <cat@gensokyo.uk>

command/node.go

@@ -3,6 +3,7 @@ package command
 import (
 	"flag"
 	"io"
+	"strings"
 )
 
 type node struct {
@@ -12,6 +13,9 @@ type node struct {
 	out  io.Writer
 	logf LogFunc
 
+	prefix []string
+	suffix strings.Builder
+
 	f   HandlerFunc
 	set *flag.FlagSet
 }