class and id attributes for img caption shortcode

The 'id' and 'class' attributes for an image caption (as can be specified by the admin) will now be output as would be expected. (Unlike WP's img_caption_shortcode function (in WP's includes/media.php file) I have intentionally not output 'wp-caption' or the align attributes in the 'class' value in case this may interfere with the FoundationPress layout.)
Merge pull request #98 from jhodgski/master

class and id attributes for img caption shortcode