Adding Filters for special page

Adding 2 new filters : 'dw_pages_types_register' and 'dw_instance_visibility', the first one is for registering "custom page" and the later one is for allowing plugin / themes to add their own custom logic for determining the widget visibility.

Use case example :
Plugin / theme has special archive page that is not related to taxonomy or post type, valid example is like user listing page. By registering the custom page to the $page_types array DW will pickup the custom page and store the checkbox value to database. Then theme / plugin can check the $instance for the saved array and perform their own visibility logic.
Merge pull request #17 from duckzland/master

Adding Filters for special page
Thank you @duckzland, looks great!