Add a way to override language strings without recompiling .po files

[pietroalbini]

Currently, the only way to override translation strings is to edit the .po files, rebuild those, and reinstall botogram with the rebuilt .mo files. That's a really bad UX, and it might cause problems with multiple bots.

The proposed API is:

bot.override_i18n = {
    "No description available": "Hey, that's empty!",
}

Meta issue: #46

[pietroalbini]

Fixed by #91