Feature request: Support offset for rdsquashfs

Feature request: Support offset for rdsquashfs, like unsquashfs does: ``` [-o BYTES, -offset BYTES] skip BYTES at start of FILESYSTEM. Optionally a suffix of K, M or G can be given to specify Kbytes, Mbytes or Gbytes respectively (default 0 bytes). ``` https://manpages.debian.org/testing/squashfs-tools/unsquashfs.1.en.html#o