replace usage of _.extend with Object.assign

src/dig.js

@@ -7,8 +7,7 @@ exports = module.exports = {
 const assert = require('assert'),
     constants = require('./constants.js'),
     dns = require('dns'),
-    safe = require('safetydance'),
-    _ = require('underscore');
+    safe = require('safetydance');
 
 // a note on TXT records. It doesn't have quotes ("") at the DNS level. Those quotes
 // are added for DNS server software to enclose spaces. Such quotes may also be returned
@@ -20,7 +19,7 @@ async function resolve(hostname, rrtype, options) {
 
     const defaultOptions = { server: '127.0.0.1', timeout: 5000 }; // unbound runs on 127.0.0.1
     const resolver = new dns.promises.Resolver();
-    options = _.extend({ }, defaultOptions, options);
+    options = Object.assign({}, defaultOptions, options);
 
     // Only use unbound on a Cloudron
     if (constants.CLOUDRON) resolver.setServers([ options.server ]);