nsq_to_file: updates

[judwhite]

See PR #794 for discussion.

• --topic and --topic-pattern should be mutually exclusive options
• --topic and --topic-pattern should work for both nsqd and nsqlookupd
• Report unrecognized arguments, don't silently ignore them; see flag.Args

[mreiferson]

The second item on this was completed by the fact that we fatally exit if nsq_to_file is provided no --topic or --lookupd-http-address arguments.

The last item is being addressed in #1117.

Closing!