Allow for passing in additional order fields

This allows for the caller to add notBefore/notAfter for servers that support them, or the "replaces" field which is added in draft-ietf-acme-ari-04 for servers that support ARI.
(Again, not sure if there might be a better approach.)

acme.js

@@ -1159,9 +1159,8 @@ ACME._orderCert = function (me, options, kid) {
 		// raw wildcard syntax MUST be used here
 		identifiers: options.domains.map(function (hostname) {
 			return { type: 'dns', value: hostname };
-		})
-		//, "notBefore": "2016-01-01T00:00:00Z"
-		//, "notAfter": "2016-01-08T00:00:00Z"
+		}),
+		...options.additionalOrderFields
 	};
 
 	return ACME._prepRequest(me, options)