Add "maximum size [in kB]" to reducing strategies

[kryger]

I'd be happy to see an option to reduce my images until the file size reaches <X kb. Some services have a hard limit on uploaded image, if I recall correctly Gmail also doesn't count images smaller than 500kb against one's Google Drive allowance.

This could be achieved by trial-and-error: resize/compress and retry if the file is still >X. The user could also add the range of resolutions/compression rates they can live with, i.e. prefer more compressed but larger images.