WIP Adds ability to set custom taxonomy terms on create and edit post

I couldn't get the `tax_input` paramenter of `wp_insert_post` to work reliably for some reason. As we already have the post ID at this point, `wp_set_object_terms` works fine.

This expects an array of term names.

`wp_set_object_terms` can also use an array of IDs or slugs, or just an ID or a slug, which should probably be handled also.

- [ ] Detect if tern names or IDs were passed, for sanitizing
Rather than nesting this deeply, I'd rather flip the check and `continue` if the taxonomy doesn't exist. :)