[Bug-readline] For users of the "vi" keymap, please enable "show-mode-in-prompt" by default

[Jason Spiro]

The new bash 4.3 and readline 6.3 include a new user-settable
variable:  "show-mode-in-prompt".  It was designed for people who use
readline's vi keymap.  It adds a character to the beginning of the
prompt, showing whether readline is in "command mode" or "insert
mode".

Thank you for adding this useful feature to readline.  Unfortunately,
it's off by default.  And so only those who know about the feature
will get to take advantage of it.

I would imagine that the vast majority of vi keymap users will want it to be on.

I wonder if you could please enable it by default?  This way, anytime
that a user enables readline's vi keymap, the mode indicator will
instantly appear.  Anyone who doesn't want it can edit their
~/.inputrc file and disable it themselves.

P.S.  Thank you very much for maintaining bash and readline --
highly-featureful, well-documented software packages that numerous
people rely on every single day.

Kind regards,
-- 
Jason Spiro:  computer consultant.
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