enter_accept will make Atuin immediately accept an execute a command
when selected. It defaults to false in our binary, but the default
config enables it.

This means that users who already use atuin will not default to the new
behaviour unless they opt in, but new users will have it by default.

Thanks to @davidhewitt for the patch and bulk of this implementation!
Currently we have it just for zsh, but I'll follow up with other shells
(unless anyone beats me to it :D)

Co-authored-by: Conrad Ludgate <conradludgate@gmail.com>