Preserve aspect ratio when generating with only one of -w/-h

[jhrcek]

Use case: I want to generate an animation with a script based on reanimate.
I want to specify just one dimension, so I run

./script.hs render -w 100

Expected behavior:
the height is calculated so that aspect ratio of the animation is preserved AND I don't have to do manual calculation to determine how much I should shrink the height to preserve aspect ratio.

Actual behavior:
the height remains at default value (1440) so the output looks like 1440x100 noodle :-)

Would you agree that if exactly one of -w / -h is provided, the other should be recalculated? If yes, I can do a PR implementing that.

[lemmih]

Agreed, no one wants a noodle.