Invalidate importlib caches if command is makemigrations

[bckohan]

some commands generation additional python files. importlib caches directory reads which means if you run makemigrations followed by migrate in the same process, migrate will not find the new migration files. This can be fixed by a call to importlib.invalidate_caches(). Users could do this themselves using pre and post hooks (#9) but makemigrations -> migrate is probably a common enough flow that it warrants handling this special case internally automatically.