Preserve aspect ratio during resizing when the user provides both width and height

[mdesantis]

Hello!

First of all, thanks for this simple yet powerful utility :-)

I implemented aspect ratio preservation during resizing when the user provides both width and height dimensions. This way, using imgcat with tools like fzf is just wonderful, for example:

fzf --preview='imgcat --width=$FZF_PREVIEW_COLUMNS --height=$(expr $FZF_PREVIEW_LINES \* 2) --half-height --depth=24bit {}'

I added the new CLI option -P|--no-preserve-aspect-ratio to disable it. I made it default to true because it doesn't make much sense to me to shrink an image before viewing it, but it's a breaking change. We may want to make it opt-in instead of opt-out if we care about backward compatibility.

Finally, I ensured height to be greater than zero after resizing, just like it happens for width.

[eddieantonio]

Excellent work! LGTM!