apps/nsq_to_nsq: add basic message filtering options

adds --require-field, --require-value, and --whitelist-field options

This code compiles, but I haven't tested in in exactly this form - I'm working in an environment where the version of go-nsq that's installed is one just before the reader-options refactor. I'll give this some proper testing someday soon, but in the meantime I invite a critique of the code and/or suggestions that I stuff it somewhere else.