Include GIF handling in image processing script

[adamduncan]

Currently we only support processing of jpg, png and svg.

We also need to support gif. I expect we'll need to update the scripts/images.js build task to include this extension, and make use of imagemin-gifsicle or similar.

See current PR in need of this support: #236

[Pete-Robelou]

All done.

I set the number of colours to 96, this offers a reasonable level of compression without degrading the image too much. Our mileage will vary a bit though depending on the image, ideally we need to encourage the use of jpg for photography and png for everything else.

There is also webp but that's a discussion for another day 🙂