Allow to enable link preview.

Hi, currently rssbot sends all notifications without link previews. I think it would be great if we could keep this as the default and allow users to enable link previews manually, for example:

- Enable Link Preview

```
/preview https://example.com/feed.xml
```

- Disable Link Preview

```
/unpreview https://example.com/feed.xml
```

All other behaviors would remain unchanged.