Add new 'defaults' hook for submodules

In order to ensure that sites can use hook_metatag_config_default_alter() to override all defaults provided by Metatag, add a new hook for defaults provided by Metatag's submodules that append to the hook_metatag_config_default() provided by the main Metatag module, so that any modules implementing hook_metatag_config_default_alter can be assured they'll execute after all of the submodules have loaded their defaults already.