Add $unique param to add_{$meta_type}_meta actions

[barryceelen]

The hooks add_{$meta_type}_meta and similar provide context about $object_id, $meta_key, $_meta_value and such, but do not tell whether the meta data added or updated is supposed to be $unique.

Adding this information could be helpful. My use case for instance is propagating meta data across posts.

[barryceelen]

In a similar fashion, the delete_{$meta_type}_meta could have the $delete_all param added.