allow spider to limit response size to make the broad crawl more robust

implement `get_response_size_limit` in spider to limit the size per request. e.g.

    def get_response_size_limit(self, request):
        if self.is_safe_url(request.url):
            return 16 * 1024 * 1024
        return 1 * 1024 * 1024

otherwise no limit on the response size.