refactor to use Promises and async/await

This introduces slightly breaking changes to the signature of
s3rver.run() and s3rver.close() since they can now return promises if no
callback is provided.

.eslintrc

@@ -4,7 +4,7 @@
     "es6": true
   },
   "parserOptions": {
-    "ecmaVersion": 2017
+    "ecmaVersion": 2018
   },
   "extends": [
     "eslint:recommended",

bin/s3rver.js

@@ -54,12 +54,12 @@ if (program.key && program.cert) {
   program.cert = fs.readFileSync(program.cert);
 }
 
-new S3rver(program).run((err, host, port) => {
+new S3rver(program).run((err, { address, port }) => {
   if (err) {
     // eslint-disable-next-line no-console
     console.error(err);
     process.exit(1);
   }
   // eslint-disable-next-line no-console
-  console.log("S3rver listening on %s:%d", host, port);
+  console.log("S3rver listening on %s:%d", address, port);
 });

lib/app.js

@@ -27,10 +27,10 @@ module.exports = function(options) {
   );
 
   app.use((req, res, next) => {
-    const host = req.headers.host.split(":")[0];
+    const [host] = req.headers.host.split(":");
 
     // Handle requests for <bucket>.s3(-<region>?).amazonaws.com, if they arrive.
-    const bucket = (/(.+)\.s3(-.+)?\.amazonaws\.com$/.exec(host) || [])[1];
+    const [, bucket] = /(.+)\.s3(-.+)?\.amazonaws\.com$/.exec(host) || [];
     if (bucket) {
       req.url = path.join("/", bucket, req.url);
     } else if (