More new hooks: custom_metadata_manager_init_metadata.

This will allow users of the plugin to optimize their field/group registration only in the contexts where necessary instead of admin_init.

Update examples and docs as well.
No need to pass $this as a reference in callbacks.
jquery-ui-datepicker and chosen-js are dependencies for the custom metadata js.
Activate over-protective mode: escape all the things.
Use esc_textarea to escape the textarea