Extension: add support for tgz

[figsoda]

Fixes #47

[marcospb19]

Thanks for opening a pull request! We really appreciate your help.

[figsoda]

I did a force push and added a new Tgz format that emulates [Gzip, Tar] and should give the correct error messages

[marcospb19]

If you don't mind, I did some small changes (added a comment here and created an extra variable there).

Was trying to remove some of the code repetition in the compress_files, but I realized it was not possible, but I committed the small changes anyways.