format: hostname is too restrictive #129
Description
Currently the regexp ^[_a-z]+.([_a-z]+.?)+$ is used to validate hostnames.
However hyphens should be allowed in the middle, also digits.
Perhaps see http://stackoverflow.com/questions/106179/regular-expression-to-match-dns-hostname-or-ip-address for better regular expressions usable for this.
There are multiple RFCs with slight differences in what is allowed. Unfortunately json-schema refers to RFC 1034, which does not actually define what is permissible in a host name at all, other than that it contains dots (and maybe not even that if it is relative and consists of only one label).