Add map_meta_cap filter

[rmccue]

I was recently creating users with custom capabilities and noticed the edit_post, etc, family translated by map_meta_cap. I needed this functionality, but it's currently missing a filter.

I hereby propose this filter:

[rmccue]

Add filter to wp-includes/capabilities.php

[westi]

(In [10874]) Let a plugin filter the expanded capabilities returned by map_meta_cap. Fixes #9462 props rmccue.