minor quirks in chat w/ new readline functionality

[roblg]

Saw the readline addition this week and wanted to check it out. Can confirm that things like arrow keys, alt+delete (on macOS) now work (yay!). But there are some weird interactions between (I think) readline and click prompts.

Quirks:

• When using 'alt+delete' to delete the last word on a line, the space character of the prompt is also deleted.
  

• Accessing history with Ctrl+r is allowed and functional, but breaking out w/ Ctrl+c causes click prompt to abort. Exiting with Esc deletes the line including the click prompt.
  

I almost didn't file these because they're so, so nitty, but the readline change is fresh so I thought it would be better to do it now than later. 😃

Also, thanks for the tool and the extensive docs and blogs -- it's been really helpful, and a lot of fun. 🙇‍♂️

[yorickvP]

Should be fixed by pallets/click#2969