Add taxonomy term helpers

WP is absolutely horrible with taxonomies. Just look at wp_set_object_terms / wp_set_post_terms.

Add helpers that allow users to add terms to posts without supplying data that isn't even needed: 

`\rnb\terms\add($term_id, $object_id)`

All terms live in the same table, having to supply taxonomy is just ridiculous because IDs are unique. 