#13 add -offset option to skip <bytes> at start of input file

[Yury Siamashka]

I use unsquashfs to extract files from disk images. I don't have
squashfs image but the whole disk image. That is why I need to skip
some bytes to point where my squashfs image is located.

So I suggest to add -offset option to skip <bytes> at start of input file.

[Yury Siamashka]