Add draft-07 formats: iri

[jcoyne]

http://json-schema.org/draft-07/json-schema-release-notes.html#formats

I'm particularly interested in iri

[santhosh-tekuri]

"net/url/url.go" in standard lib implements RFC 3986
IRI requires RFC 3987

but it seems, url.go seems to work for IRI in most cases.

so you can simply register "uri" as "iri" as below:

fun init() {
    formats.Register("iri", formats.IsURI)
}