mod: add go.mod

go.mod

+module howett.net/dnsup
+
+go 1.14
+
+require (
+	github.com/miekg/dns v1.1.29
+	golang.org/x/crypto v0.0.0-20200403201458-baeed622b8d8 // indirect
+	golang.org/x/net v0.0.0-20200324143707-d3edc9973b7e // indirect
+	golang.org/x/sys v0.0.0-20200331124033-c3d80250170d // indirect
+	gopkg.in/yaml.v2 v2.2.8
+)