Progressive JPEG support

[SergeyKorochansky]

Currently Mogrify doesn't support generation of progressive JPEG images. It would be nice to add option to specify interlacing scheme.

[talklittle]

For now, does Mogrify.custom(image, "interlace", "line") work as expected for you?

ImageMagick has so many command line options, we need to decide a better way to organize them in this library. I'm opening another issue to track this general problem. (#34)