Added hot reloading capabilities

[mpfau]

Hi there,
I just added hot reloading capabilities to this plugin (see https://github.com/capaj/systemjs-hot-reloader).
Precondition is, that you wait for the initialization of the hot reloader and that the hot reloader is present at System.hotReloader.

Therefore, the bootstrapping should look like:

var bootstrap = Promise.resolve()
if (location.origin.match(/localhost/)) {
    System.trace = true
    bootstrap = System.import('capaj/systemjs-hot-reloader').then(function(HotReloader){
        System.hotReloader = new HotReloader.default('http://localhost:9080')  // chokidar-socket-emitter port
    })
}
bootstrap.then(function() {
    return System.import('src/app.js')
})