Plugin hook enclosures

Adds a plugin hook in the format_article_enclosures() function. This allows plugin developers to generate their own HTML content for enclosures and/or filter the array of enclosures.

This addresses a user request in the forums here: [HOOK to remove content from external servers](http://tt-rss.org/forum/viewtopic.php?f=8&t=2836&p=16664)