Support MPO

Almost all of my jpeg pictures from various cameras aren't compressing: ``` picopt -r * Didn't optimize any files. ``` Stepping through the code the file is being recognized by PIL as MPO. These are jpeg files with a smaller photo embedded as a thumbnail, commonly done by cameras to speed viewing. Here's some discussion: https://github.com/python-pillow/Pillow/issues/1138 1. Can picopt be updated to compress these jpg files? I have tried jpegrescan and it successfully compresses the files. 2. Figuring this out was a PITA because the error message is not helpful: ``` picopt -r P1000223.JPG Didn't optimize any files. ``` Can picopt better describe why it won't optimize the file, at least when a single file targeted? Maybe something like one of "Not an image or "Image type foo not recognized"