Fix #1: Allow easier specification of output size

[liquidcake1]

Turned out to be a bit of an epic. I'm not sure of behaviour in other terminals, but I've tested this in Konsole.

Fixes #1

[liquidcake1]

Tested with:

for dims in "50x20" "200x20" "400x20"; do
  convert -size "$dims" xc:white white.png
  for args in "" "--width=100%" "--width=original" "--width=100% --height=auto" "--width=100% --height=default" "--width=auto --height=original"; do
    echo "$dims $args"
    python3 -m imgcat white.png $args
  done
done

As you can see, default (ie. not passing the argument) appears to be the same as auto, though this isn't actually documented on the protocol page I found. Resizing my terminal to be small in the vertical axis and fiddling appropriately with the parameters got a broadly similar behaviour, but one which is harder to screenshot.