perf(plook): use custom HTTP agent

By using a custom HTTP agent, we are able to increase the maxSockets.
This allows for much greater performance when doing ApacheBench
tests, for example.

lib/plook.js

@@ -34,6 +34,10 @@ function Plook( root ) {
         return new Plook();
     }
 
+    // Create a custom socket for when getting files
+    this._agent = new https.Agent();
+    this._agent.maxSockets = 10;
+
     Object.defineProperty( this, "_cache", {
         value: root instanceof Plook ? root._cache : LRU({
             max: 500
@@ -193,6 +197,7 @@ Plook.prototype.get = function( name, version, file, etag ) {
 
     promise = this.findURLs( name, version, file ).call( "map", function( reqUrl ) {
         var options = url.parse( reqUrl );
+        options.agent = plook._agent;
         options.headers = {
             "if-none-match": etag
         };