nsq_to_nsq: need --writer-opt flag

`nsq_to_nsq` has no means to configure the various `go-nsq` `Config` flags on the output side (it only has `--reader-opt`).

I'm proposing a few things:

- [x] add `--writer-opt` flag to configure the `Producer`
- [x] accept `--consumer-opt` as an alias for `--reader-opt`
- [x] accept `--producer-opt` as an alias for `--writer-opt`
- [x] ~~add an exhaustive list of options + help text in the `--help` usage output for all the various options~~
- [x] update docs on http://nsq.io/components/utilities.html
Although it was a misnomer, removing reader-opt (even though it was used for the write config) is a breaking api change.