reduced usage of commands (using 7z, and tar now), refactored code for arg spec and OS check

As the title suggests, I've replaced the use of commands for the tarballs, and for zips. We default to using `7z` and `tar` now. Regardless, the other commands are still visible as fallbacks. Also, I've refactored the code for the `archive_commands` table, and the OS check. Solves #9