Add support for pausing, aborting, and promisifying appdmg execution

[argv-minus-one]

This PR adds some new methods and properties to the object returned by the appdmg function (that is, the pipeline object):

---

ee.waitFor(promise)

Pauses execution until the given Promise completes. If the promise rejects, then the appdmg run is aborted. This lets you do custom asynchronous work on the disk image while it's being built.

For example, suppose your disk image will contain a folder called “Super Secret Folder”, which you want to be hidden from the Finder. Here's how to do it, using the Xcode command-line tools:

const appdmg = require('appdmg');
const execa = require('execa');
const path = require('path');
const ee = appdmg({
  // appdmg options go here
});
async function hideSecretFolder () {
  // Use the SetFile program (it comes with Xcode) to hide `Super Secret Folder` from the Finder.
  await execa('SetFile', [
    '-a',
    'V',
    path.join(ee.temporaryMountPath, 'Super Secret Folder')
  ]);
}
ee.on('progress', info => {
  if (info.type === 'step-begin' && info.title === 'Unmounting temporary image') {
    ee.waitFor(hideSecretFolder());
    // appdmg will now wait, until hideSecretFolder() is finished, before unmounting the temporary image.
  }
})

ee.abort(err)

Abort the appdmg run with err as the reason. It must be a truthy value, preferably an Error.

ee.asPromise

A Promise that completes when appdmg is finished.

ee.temporaryImagePath

Path to the temporary disk image. This is a writable disk image that appdmg creates and mounts while it's working.

ee.temporaryMountPath

Path where the temporary disk image is currently mounted. This property is set when it's mounted, and deleted when it's unmounted.