[chore] Validate/set account domain (#619)

* add miekg/dns dependency

* set/validate accountDomain

vendor/github.com/miekg/dns/version.go

@@ -0,0 +1,15 @@
+package dns
+
+import "fmt"
+
+// Version is current version of this library.
+var Version = v{1, 1, 49}
+
+// v holds the version of this library.
+type v struct {
+	Major, Minor, Patch int
+}
+
+func (v v) String() string {
+	return fmt.Sprintf("%d.%d.%d", v.Major, v.Minor, v.Patch)
+}