use WP HTML5 markup for captions instead of Soil

Use `add_theme_support` to enable native HTML5 captions and filter the
caption shortcode to add in the caption class from Bootstrap.

There is a potentially simpler way to achieve this by adding Bootstrap caption styles to `wp-caption-text` class and not filtering the caption shortcode. Also not sure if you want this to live in extras.php.

To complete this merge, the `soil_caption` function needs to be removed from Soil in `modules/clean-up.php` per the discussion at roots/soil/issues/25