handle --help for subcommands

If subcommand is provided (and exists) and its first arg is -h/--help,
go ahead and intercept the call; redirecting to rbenv-help <subcommand>

This means subcommands and plugins need not handle --help flag
themselves

libexec/rbenv

@@ -111,6 +111,10 @@ case "$command" in
   fi
 
   shift 1
+  if [ "$1" = --help ]; then
+    exec rbenv-help "$command"
+  else
     exec "$command_path" "$@"
+  fi
   ;;
 esac