Add option to skip static localhost entries in the final hosts file.

Add option to skip static localhost entries in the final hosts file.

This is particularly handy when you want to add the generated hosts file
as additional static 'hosts' file in DNS servers without affecting the default
`/etc/hosts`.

For example, `--addn-hosts=<file>` in dnsmasq [1] can be used exactly for this
purpose.

[1] http://www.thekelleys.org.uk/dnsmasq/docs/dnsmasq-man.html