[request] better automatic resize

[nycki93]

I always end up having to pull out a calculator to work out how to resize my image to fill a full map (or a full 2x2 of maps, etc) without stretching or distorting it. It would be convenient if this was a built in option. For example:

/image2map create 1m auto https://crouton.net/crouton.png

should be equivalent to

/image2map create 128 120 https://crouton.net/crouton.png

where "1m auto" is short for "1 map wide, automatic height".