Allow filtering response before returning

This will allow plugins to modify the response based on the request.
Initial extras.php commit
Deleting JS Loader from Plugin.php

Moved to extras.php
Doc changes for the new extras.php
Moving action to top of extras.php
Splitting a bunch of functions into extras.php
Taking some more stuff out of plugins.php

Went into extras.php
Small changes to initial extras.php

No newline & “//“ comments
Ensure `$result` is always a response object
Update routes.md

posts_per_page is actually a public query variable
Initial pass at get_items and response for users controller.
Included users controller file and added users collection route.
Initial pass at get_item for User controller.
Register route for User get_item.
Add create method to base controller.
Add create users route to initial registered routes.
Fixed $request parameter for get_items method in users controller.
Correct logic when building response for get_items in user controller.
Initial pass at create a user logic in the user controller.
Add route to edit a user to initial registered routes.