upgrade version to 0.4

Merge branch 'readline_keys'

release 0.3.1

use $SHELL instead of hardcoded bash

Fixes #2.

This currently requires the $SHELL to support a `-c` option. For shells
that do not support such option, it's recommended to write a wrapper
script.

Thanks to everybody who contributed ideas how to resolve this, and
especially for ottidmes & symphorien on #nixos IRC channel.

fix restart on Ctrl-Q

Broken after changing the logic for "Enter to run" feature.

et al.

readme: emphasize keyboard shortcuts