Reading multiple files

**Is your feature request related to a problem? Please describe.**

I maintain several plugins based on ndarrays (https://www.npmjs.com/package/ndarray). It works well but I find the Jimp api quite appealing and am thinking of porting the plugins to Jimp.

I was wondering though, most of my plugins deal with multiple images at the same time. For example, averaging a certain amount of images, etc. But I don't think that the Jimp read method accepts an array of images right?

**Describe the solution you'd like**

It would be nice to have a way to work with a collection of images. Not serially, but the ability to operate on all of them at the same time (as for averaging you'd need all the image data available at the same time).

**Describe alternatives you've considered**

I've not considered any alternatives yet, but advice from the maintainers on how best to do this with the existing API would be welcome as well.

**Additional context**

None atm.