docs(plunkers): prepare plunkers for the quickstart reboot (#2759)

* plunker builder removes files and scripts not needed in plunkers (e.g., systemjs.config.js)
* no longer adds placeholder README.md (deemed a distraction)
* tag is now "Angular" instead of "Angular2"
* new plnkr.json "includeSystemConfig" option adds systemjs.config.js back; needed for testing plnkrs.
* incidentally changes comment "Polyfill(s)" to "Polyfills" in all index.html

public/docs/_examples/quickstart/ts/index.html

@@ -10,7 +10,7 @@
     <!-- 1. Load libraries -->
     <!-- #docregion libraries -->
     <!-- #docregion polyfills -->
-     <!-- Polyfill(s) for older browsers -->
+     <!-- Polyfill for older browsers -->
     <script src="node_modules/core-js/client/shim.min.js"></script>
     <!-- #enddocregion polyfills -->