In ASA URL filtering can be configured for one url at a time.you have to configure for one URL after the other.I think that a whitelist can configured only using an external server.URL filtering is CPU-intensive, the use of an external filtering server ensures that the throughput of other traffic is not affected. However, based on the speed of your network and the capacity of your URL filtering server, the time required for the initial connection can be noticeably slower when traffic is filtered with an external filtering server.