Add command line option to replace . with _ in hostname

If ran with --underscores, it will change the graphite output so that the hostname has underscores rather than dots.  This makes "folders" in graphite a lot cleaner.

Example without --underscores:
es.es1.subdomain.domain.com.jvm.gc.collector.ParNew.count 2075 1359133887
Example with --underscores:
es.es1_subdomain_domain_com.jvm.gc.collector.ParNew.count 2075 1359133887