Add DNS over HTTPS (doh) client example. (#32)

Fixes #30.

examples/doh.js

@@ -0,0 +1,53 @@
+
+'use strict'
+
+/*
+ * Sample code to make DNS over HTTPS request using POST
+ * AUTHOR: Tom Pusateri <pusateri@bangj.com>
+ * DATE: March 17, 2018
+ * LICENSE: MIT
+ */
+
+const packet = require('..')
+const https = require('https')
+
+function getRandomInt(min, max) {
+  return Math.floor(Math.random() * (max - min + 1)) + min;
+}
+
+const encodedPacket = packet.encode({
+  type: 'query',
+  id: getRandomInt(1, 65534),
+  flags: packet.RECURSION_DESIRED,
+  questions: [{
+    type: 'A',
+    name: 'google.com'
+  }]
+})
+
+const options = {
+  hostname: 'dns.google.com',
+  port: 443,
+  path: '/experimental',
+  method: 'POST',
+  headers: {
+    'Content-Type': 'application/dns-udpwireformat',
+    'Content-Length': Buffer.byteLength(encodedPacket)
+  }
+}
+
+const request = https.request(options, (response) => {
+  console.log('statusCode:', response.statusCode)
+  console.log('headers:', response.headers)
+
+  response.on('data', (d) => {
+    console.log(packet.decode(d))
+  })
+})
+
+request.on('error', (e) => {
+  console.error(e)
+})
+request.write(encodedPacket)
+request.end()
+