[System.js branch] Externalize systemjs config to a separate file

[jeeyun]

We should externalize systemjs config into a separate file and include it in the index.html. Also let's investigate if we can consolidate this config and use it in both dev and prod mode. Current setup requires updates in two places (index.html and gulpfile.js) if systemjs setting changes. It might be worthwhile to rip out systemjs builder all together and use rollup or some other bundler for the prod build.