Image proxy user-agent

[janboddez]

Some site return a 403 when WordPress attempts to fetch their images. Should be okay with a "spoofed" user agent.

[janboddez]

We can make this a setting or a filter or, perhaps better for this type of config thing, a global constant that should go in wp-config.php.

[janboddez]

Filterable using the feed_reader_user_agent hook.