Allow masking some paths or apps

[jayvdb]

I am using django-oscar-api, and it includes lots of DRF endpoints which do not have a trailing /, and I cant add them.

e.g.

?: (url_checks.E001) URI pattern does not end with slash
	HINT: Add slash to end of path "baskets/<int:basket_pk>/lines/<int:line_pk>/lineattributes/<int:pk><drf_format_suffix:format>", named "lineattribute-detail"

It would be nice to be able to somehow mask these ones, by namespace, path prefix or app name.

[jayvdb]

Also a config option to drop the error to a warning (globally) would be one way to let users get the check into place even if there are still some pre-existing problems, and then slowly fix any problems which might involve template changes, or a frontend-app change, or as noted above, change in a third-party app.

[jayvdb]

I should also be able to only run it on certain apps
manage.py check [app_label [app_label]]

[jayvdb]

I just built my first check, which does at least the last two bits - per-app checks, and a setting which toggles between error/warning