command: hard wrap help message flags

This greatly improves readability when lots of flags are registered.

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

command/help.go

@@ -1,6 +1,7 @@
 package command
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"io"
@@ -13,14 +14,31 @@ var ErrHelp = errors.New("help requested")
 func (n *node) PrintHelp() { _ = n.writeHelp() }
 
 func (n *node) writeHelp() error {
+	prefix := strings.Join(append(n.prefix, n.name), " ")
+	var buf strings.Builder
+	offset := 7 + len(prefix)
+	line := offset + 14
+	w := bytes.Repeat([]byte{' '}, offset+1)
+	w[0] = '\n'
+
+	for _, flag := range n.suffix {
+		line += len(flag) + 1
+		if line >= 80 {
+			line = offset + len(flag) + 1
+			buf.Write(w)
+		}
+		buf.WriteByte(' ')
+		buf.WriteString(flag)
+	}
+
 	if _, err := fmt.Fprintf(n.out,
-		"\nUsage:\t%s [-h | --help]%s COMMAND [OPTIONS]\n",
-		strings.Join(append(n.prefix, n.name), " "), &n.suffix,
+		"usage: %s [-h | --help]%s <command> [<args>]\n",
+		prefix, &buf,
 	); err != nil {
 		return err
 	}
 	if n.child != nil {
-		if _, err := fmt.Fprint(n.out, "\nCommands:\n"); err != nil {
+		if _, err := fmt.Fprint(n.out, "\ncommands:\n"); err != nil {
 			return err
 		}
 	}