Extending the GNU readline support

[cmacrae]

Hey @c-bata :wave: Thanks so much for kube-prompt. I use it every day and love it :ok_hand:

It'd be nice if it supported the 'Alt' key set of shortcuts from GNU readline.
As most others are already supported (Ctrl+A, Ctrl+E, Ctrl+K, etc.), I figured this would be a reasonable request.

I'd love to contribute myself and submit an MR for this. In fact this is something I wanted to include in my contributions toakavel/up a while back, but couldn't quite work out how to do it.

Perhaps if you can spare a moment, you might be able to hint/point me in the right direction? :pray:
I'll have a look over the code and see where I get to, but I wanted to ask first if this is a contribution you'd be happy with?