Handle more IF service URLs

We could parse a Borogove.app game URL for the game file. (Look for `default_story` in a `<script>` tag. It's a big ugly URL but it's fetchable.)

Same goes for an Inform web site. (`default_story` might have to be base64-decoded.)  Or look for

```
<script src="interpreter/Advent Door.gblorb.js" type="text/javascript"></script>
```

On itch.io, pull the iframe and try the same on that.
