Handle Unicode in DNS labels

[srsudar]

A comment on #7 observes that we should be supporting internationalized DNS labels and unicode in general in the DNS layer.

This isn't strictly necessary for SemCache functionality, as we can enforce ASCII characters and control DNS records, but especially if we continue to allow user-specified .local domain names we should support non-ASCII characters.

I haven't yet found the canonical way DNS handles labels, but it obviously exists. Here are several relevant RFCs that will prove useful when this handled. It sounds like it might be Punycode, though some of the things I've come across have implied this is outdated.

RFC 4343
RFC 3454
RFC 3492

This looks like a promising npm module for handling Punycode.