Rebuild a Debian ISO with preseed/custom files.

# Usage

    rebuild-debian-iso <input-iso> <output-iso> <preseed-file> [file-dir]

    input-iso:    the debian iso to modify
    output-iso:   where to store the modified iso
    preseed-file: the location of a preseed file to inject
    file-dir:     an optional directory to be put at extra/ in the iso