Fixes#2493
No need to dual boot `AppComponent` and `ProvidersComponent` only to
ensure that we could demo optional injection; instead just inject null
for the `@Optional` constructor parameter type.
No prose is affected by these changes.
* 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
Patrice Chalin
Ward Bell
cam73
Pascal Precht