Get operation progress information with handler function

Nobody has claimed this yet.

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 :
```js
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 ?

Start with the sharp() transformation pipeline and the proposed 'progress' handler, then investigate how libvips' VIPS_PROGRESS environment variable reports operation progress. Define what progress should represent for operations such as crop, resize, and tiles, and confirm the API and behavior needed before considering the feature complete.

Written by the indexing model from the issue text.