Options and Documentation Fixes

- Correct README flag for `--cache_dir`
- Correct external DNS flag (`--server_address, -a`) flag in usage message
- Remove duplicate option descriptions, configure as [Optimist aliases](https://github.com/substack/node-optimist#aliaskey-alias)
- Remove custom option defaults, configure as [Optimist defaults](https://github.com/substack/node-optimist#defaultkey-value)
- Add descriptions for `--help, -h`, `--version, -v`, and `--log_level` options