Get operation progress information with handler function #2189

First of all, thanks for the great job done with sharp !!! It is a very powerful tool for images manipulation...

I would like to get some information about progression (percentage or weight) of current image operation (eg: crop, resize, tiles...)

I didn't found anything about this need in all existing issues...

I imagine a handler function injection like :

var transformer = sharp()
  .resize(300)
  .on('progress', function(progress) {
    console.log('Processing: ' + progress.percent + '% done');
  });

I know libvips consider an environment variable named VIPS_PROGRESS, so I guess there is something to do ?