Add support for StuffIt X base-N encoding

[ci-macpaw]

Originally reported on Google Code with ID 421

andrews05:

No error reported, the progress window appears for a moment and then vanishes.

---

* *Attachment: [enrle.sitx](https://storage.googleapis.com/google-code-attachments/theunarchiver/issue-421/comment-0/enrle.sitx)*

[ci-macpaw]

paracelsus:

This file seems to be encoding using the "ASCII" encodings in StuffIt X. These are not
supported yet.

I have updated the code to properly report that the file is unsupported, rather than
exiting mysteriously. It will probably be some time before I have the time to get around
to actually adding support for these encodings, though.

[ci-macpaw]

andrews05:

Using The Unarchiver 3.0a2.