Added HOOK_FORMAT_ENCLOSURES plugin hook.

Runs HTML and enclosures array through a plugin hook when rendering an article's enclosures in format_article_enclosures(). Allows plugins to override handling of how enclosures are presented by either filtering the array of enclosures, or generating the HTML to add to the article content.
Changed how the enclosures array is checked for content so an empty array is not processed. Fixes a change from the previous branch commit.
Merge pull request #374 from dzaikos/plugin-hook-enclosures

Plugin hook enclosures