Allow passing in agent

I needed this so that I could use https://github.com/TooTallNate/node-socks-proxy-agent to configure a Socks proxy to access our WPT instance.

Author: gravityrail

lib/webpagetest.js

@@ -56,7 +56,7 @@ var filenames = {
 };
 
 // GET helper function
-function get(config, pathname, proxy, callback, encoding) {
+function get(config, pathname, proxy, agent, callback, encoding) {
   var protocol,
       options;
 
@@ -93,6 +93,10 @@ function get(config, pathname, proxy, callback, encoding) {
   if (encoding !== 'binary') {
     options.headers['accept-encoding'] = 'gzip,deflate';
   }
+    
+  if (agent) {
+    options.agent = agent;
+  }
 
   return protocol.get(options, function getResponse(res) {
     var data, length,
@@ -191,7 +195,7 @@ function api(pathname, callback, query, options) {
   } else {
 
     // make the real API call
-    get.call(this, config, pathname, options.proxy, function apiCallback(err, data, info) {
+    get.call(this, config, pathname, options.proxy, options.agent, function apiCallback(err, data, info) {
       if (!err) {
         try {
           if (options.parser) {