add support for tar.gz files

[milahu]

or document this limitation (and defer to ratarmount)

$ tarindexer -i test.tar test.tar.idx
One dot stands for 1000 indexed files.

1.0%
512.0B/50.0kB
ETA: 0s (estimate 0s)

93.0%
46.5kB/50.0kB
ETA: 0s (estimate 0s)

97.0%
48.5kB/50.0kB
ETA: 0s (estimate 0s)
done.

$ tarindexer -i test.tar.gz test.tar.gz.idx
Traceback (most recent call last):
  File "/nix/store/mwjw85jw8v79ncxip57i9w8sc2mdxi1m-tarindexer-unstable-2015-07-27/bin/tarindexer", line 123, in <module>
    main()
    ~~~~^^
  File "/nix/store/mwjw85jw8v79ncxip57i9w8sc2mdxi1m-tarindexer-unstable-2015-07-27/bin/tarindexer", line 118, in main
    indextar(dbtarfile,indexfile)
    ~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/mwjw85jw8v79ncxip57i9w8sc2mdxi1m-tarindexer-unstable-2015-07-27/bin/tarindexer", line 55, in indextar
    with tarfile.open(dbtarfile, 'r|') as db:
         ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/nix/store/sd81bvmch7njdpwx3lkjslixcbj5mivz-python3-3.13.4/lib/python3.13/tarfile.py", line 1909, in open
    t = cls(name, filemode, stream, **kwargs)
  File "/nix/store/sd81bvmch7njdpwx3lkjslixcbj5mivz-python3-3.13.4/lib/python3.13/tarfile.py", line 1785, in __init__
    self.firstmember = self.next()
                       ~~~~~~~~~^^
  File "/nix/store/sd81bvmch7njdpwx3lkjslixcbj5mivz-python3-3.13.4/lib/python3.13/tarfile.py", line 2783, in next
    raise ReadError(str(e)) from None
tarfile.ReadError: invalid header