Mogrify's -format option

[jonny-no1]

From the ImageMagick Mogrify docs:

This tool is similar to convert except that the original image file is overwritten (unless you change the file suffix with the -format option) with any changes you request.

So for the following config, mogrify will fail if the source image is of a different format than jpeg:

snowcap_im:
    formats:
        default:
            format: jpeg

It is possible that this is related to #21.

Will create a PR if I get round to it.