Version 2.4

Big pile of stuff for 2.4:

* Support for custom post type permastructs.
* Automatic integration with the [Rewrite Rule Testing](https://wordpress.org/plugins/rewrite-testing/) plugin.
* Query variables for front-end filtering via the `site_filters` argument.
* Query variables for front-end sorting via the `site_sortables` argument.
* Filters for the post type arguments and names (`ext-cpts/{$post_type}/args` and `ext-cpts/{$post_type}/names`).
* Support for outputting multiple values per key in post meta admin columns.
* The default value of the `link` argument for admin columns is now `list`.
* The `filters` argument has been renamed `admin_filters`.
* The `cols` argument has been renamed `admin_cols`.
* The `right_now` argument has been renamed `dashboard_glance`.
* Remove the backwards compatibility with pre-2.3 plural, slug, and singular arguments.