hard coded post type name

Hi Paul, thanks for the great plugin! I noticed that the post type name is hardcoded in the $parent_slug param of add_submenu_page() (cf. the “Users” and “Supported Plugins” submenus):

'edit.php?post_type=achievement'

Should be:

'edit.php?post_type=' . dpa_get_achievement_post_type()

https://wordpress.org/plugins/achievements/