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/testing/ts/1st-specs.plnkr.json

@@ -8,5 +8,6 @@
     "1st-specs.html"
   ],
   "main": "1st-specs.html",
-  "tags": ["testing"]
+  "tags": ["testing"],
+  "includeSystemConfig": true
 }

public/docs/_examples/testing/ts/app-specs.plnkr.json

@@ -19,5 +19,6 @@
     "app-specs.html"
   ],
   "main": "app-specs.html",
-  "tags": ["testing"]
+  "tags": ["testing"],
+  "includeSystemConfig": true
 }

public/docs/_examples/testing/ts/bag-specs.plnkr.json

@@ -16,5 +16,6 @@
     "bag-specs.html"
   ],
   "main": "bag-specs.html",
-  "tags": ["testing"]
+  "tags": ["testing"],
+  "includeSystemConfig": true
 }

public/docs/_examples/testing/ts/bag.html

@@ -8,7 +8,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <link rel="stylesheet" href="styles.css">
 
-    <!-- Polyfill(s) for older browsers -->
+    <!-- Polyfills for older browsers -->
     <script src="node_modules/core-js/client/shim.min.js"></script>
 
     <script src="node_modules/zone.js/dist/zone.js"></script>

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

@@ -8,7 +8,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <link rel="stylesheet" href="styles.css">
 
-    <!-- Polyfill(s) for older browsers -->
+    <!-- Polyfills for older browsers -->
     <script src="node_modules/core-js/client/shim.min.js"></script>
 
     <script src="node_modules/zone.js/dist/zone.js"></script>