fix: Expose PercyAgent as a global always (#178)

Second time is the charm right? We're going to beta test this one but
it should do it. Setting `extend: true` in the rollup config allows
the bundler to extend the global namespace (window).

So the output now looks like `this.PercyAgent = ` (where `this` is the
window object)

I cut a beta release of this and tested it on the various SDKs (btw, we need a better process for this):

- https://github.com/percy/percy-cypress/compare/rd/agent-beta-4.4
- https://github.com/percy/percy-puppeteer/compare/rd/agent-beta-4.4
- https://github.com/percy/percy-nightwatch/compare/rd/agent-beta-4.4
- https://github.com/percy/percy-webdriverio/compare/rd/agent-beta-4.4
- https://github.com/percy/percy-nightmare/compare/rd/agent-beta-4.4
- https://github.com/percy/percy-protractor/compare/rd/agent-beta-4.4
- https://github.com/percy/percy-capybara/compare/rd/agent-beta-4.4

This one should do the trick 👍

rollup.config.js

@@ -7,7 +7,9 @@ export default {
   output: {
     name: "PercyAgent",
     file: "dist/public/percy-agent.js",
-    format: "umd"
+    format: "iife",
+    // a must to add `PercyAgent` to the global namespace
+    extend: true
   },
   plugins: [
     // Allows node_modules resolution