[Req] Allow users to change the post type slug in plugin settings

Issue raised here: https://wordpress.org/support/topic/single-listing-url-currently-vehicle-how-to-change/

Would be great to add a setting to plugin settings. Additional to the code provided in the wp.org forums would be good to implement the flush_rewrite_rules() if there is a new value. This way users don't have to save the permalinks them selves in the wordpress settings.  