Sniff to enforce presence of docblock above actions and filters

A docblock should be present above all calls to the following:

* `do_action()`
* `apply_filters()`
* `do_action_ref_array()`
* `apply_filters_ref_array()`
* `do_action_deprecated()`
* `apply_filters_deprecated()`

A sniff which enforces this would be handy. Validating the docblock should probably be done separately as it's quite complex.

WPCS issue: https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards/issues/424