Add helper function to update an option

Working on WP-CLI functionality for a plugin; would be helpful to use this to update options outside of the displayed form.
since you are using the `set` method, there is no reason to take in the `$resave` argument to this helper function (because using set, it will save it to the DB). I would remove it from the cmb2_update_option function and set it to 'false'  for cmb2_options()->update.