Adding post IDs for filtering in the widget and several enhancements

Hey Eric,

I need a filter by posts for the widget, the rest of the functionality seems to be what I would need except for the filtering by post part.

I am adding a patch for adding the post IDs for optional filtering. Additionally I had to migrate to WP_Query, fix the paths (as my plugin was not in the expected directory) and move the scripts/styles to wp_enqueue_scripts due to a deprecated error on latest WPs. 

Let me know if you find these useful for an update.