Support for single metadata generation hook

Once `wpcfp_get_post_metadata ` has been introduced it would be nice to have a possibility to generate metadata field on-demand, not all from advance. It will allow to ommit field values generation if they are not required to use.

It will be usefull while implementing https://github.com/athlan/wordpress-custom-fields-permalink-plugin/issues/27.