Merge pull request #3 from cofi/custom-func

Custom for `erc-image-display-func'.
Scale image to window width or height.

Signed-off-by: Michael Markert <markert.michael@googlemail.com>
Fix condition for rescaling images.

Previous condition would have ignored non existing imagemagick support.

Signed-off-by: Michael Markert <markert.michael@googlemail.com>