Fix bug where the_content is being filtered for each call on a single post page.

On our site, we have a secondary loop in the sidebar which calls the_content(). The angellist plugin is filtering each of those posts as well, causing more output than is expected in the sidebar.

This PR checks to make sure that when `is_single()` is true, it filters only the post which is in the main query.

`is_main_query()` didn't appear to work here, so this is the best solution I could come up with.