apply_filters call allows caption shortcode to render on parts content

If 'parts' contain content, specifically images with captions, the caption shortcode [caption id='someid'] will now render the desired html. 

BEFORE: 
![screen shot 2014-07-08 at 11 46 17 am](https://cloud.githubusercontent.com/assets/2048170/3514874/492ff714-06d0-11e4-8683-6bd6c166363e.png)

AFTER: 
![screen shot 2014-07-08 at 11 45 43 am](https://cloud.githubusercontent.com/assets/2048170/3514880/5307febc-06d0-11e4-9640-779187134752.png)