allow only quality setting for image optimization

[juancresc]

As the title says, I'd like an alias/option that allows me to just decrease the quality of the image as well as use optimize commands, but without changing its size.

Right now if I don't specify a size option, I cannot generate thumbnails.

this would be useful cause I'd be able to use this lib just for image optimization.

[SmileyChris]

I'd suggest just choosing a very large image size. It won't upscale images by default so you'll get the behaviour you want.