Misc. Additions

1. WordPress Coding Standards updates
1. Added `display_posts_shortcode_post_attr` filter
1. Added `sanitize_html_class` to `display_posts_shortcode_post_class` return
1. Switched to `get_the_ID()` where appropriate
1. Added `display_posts_shortcode_wrapper` to combine `display_posts_shortcode_wrapper_open` & `display_posts_shortcode_wrapper_close`