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

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:

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

_Meta issue: #46_
