Adding several hooks to give developers more power and control over Zoninator

Actions:
* `zoninator_advanced_search_fields` - Adds the ability for developers to add fields to the zone content filters box. Also, this PR moves the existing fields to use this action so developers can remove and work around them.

Filters:
* `zoninator_recent_posts_args` - This is similar to the existing `zoninator_search_args` and allows developers to filter the results for the "Recent Posts" dropdown
* `zoninator_search_results_post` - For the search results, this filter allows developers the ability to return additional data (e.g. adding a featured image thumbnail URL)

JS Events:
* `search.request` - Fired before a search is made, allowing developers to manipulate the request arguments
* `zoninator.ajax` - Fired when a zoninator.ajax call is made (e.g. updating the recent posts dropdown), allowing developers to manipulate the request arguments