Add proxy example (#225)

* Add proxy example

* Updated after review comments

proxy-blocker/proxy/proxy-script.js

@@ -0,0 +1,21 @@
+var blockedHosts = [];
+const allow = "DIRECT 1234";
+const deny = "PROXY 127.0.0.1:65535";
+
+// tell the background script that we are ready
+browser.runtime.sendMessage("init");
+
+// listen for updates to the blocked host list
+browser.runtime.onMessage.addListener((message) => {
+  blockedHosts = message;
+});
+
+// required PAC function that will be called to determine
+// if a proxy should be used.
+function FindProxyForURL(url, host) {
+  if (blockedHosts.indexOf(host) != -1) {
+    browser.runtime.sendMessage(`Proxy-blocker: blocked ${url}`);
+    return deny;
+  }
+  return allow;
+}