Ring bell with unassigned commands in vi-mode

[elfham]

In vi-mode, readline and some implementations seem to
ring the bell with unassigned commands.
(readline can change the setting with bell-style of .inputrc)

I think Reline should follow them, so how about?

	unassigned command	ESC in command mode
readline	🔔	-
editline	🔔	🔔 (once every 2 times?)
vim	🔔	🔔
nvi	warning message	🔔
Zsh	🔔	🔔

(I think it is useful for debug of Reline itself...)