Add Custom AJAX Endpoints

[mattgrshaw]

One thing that was done in Better Search Replace to improve AJAX speed (especially when batch-processing) was to create a custom AJAX endpoint. Since with issue #83 we'll be implementing batch processing in Revisr, we should add custom AJAX endpoints for Revisr as well.

This is about twice as fast as running through admin-ajax.php, but not as fast as POSTing AJAX to a standalone PHP file and manually loading WordPress.