add unarchiver example

[jm33-m0]

Hi Matt, thanks for the great work!

I noticed there's no Unarchive function in v4 yet, and the documentation is incomplete and hard to follow, so I wrote a small program that invokes your extractor API to automatically identify the format and unarchive tarballs.

Please check the code and point out where I need to improve. It can serve as an example for users who seek a quick solution from this project.

[mholt]

Thanks for this; I like the idea of posting an example instead of prescribing one way to do it in the code itself.

I have a few suggestions on how to tweak this but I will have to come back and write those up later.

I suppose this might resolve #311.