Enable asynchronous Instantiation.

[h4cc]

Currently, Einigeln is fully synchronous. This could be changed with some additional API parameters.

Example:

di.set('foo', function(container, resolve) {
  // Not returning any value here, so Einigeln will assume resolve will be called when ready.
  setTimeout(function() {
    resolve(new Foo(container.get('bar')));
  }, 1000);
});

The additional API Call:

di.getAsync('foo').then(console.log);

For such a feature, get() needs to realize when a service used returnand when it using resolve.
So kind of a Map with a flag for each service if it is async needs to be added. Depending on that flag either get or getAsync will access each other on behalf.