Add unix permissions

[believeinlain]

Addresses issue #46

• Added file unix_mode.rs that adds metadata representing the Unix mode bits using std::fs::Permissions and std::os::unix::fs::PermissionsExt (on unix).
• Added display of the form "rwxrwxrwx" (with set uid, set gid, and sticky bits) to diff and ls commands.
• Modified tests to work and added tests specifically for the mode display functionality.
• Still some work to do adding and testing Windows compatibility (the plan is to always store archives with Unix mode bits, but to use a systematic conversion when storing/retrieving files on Windows).
• Works with old archives by setting the default mode bits to 0o100775, which should ideally be changed, as a more suitable default is 0o664 for files and 0o775 for directories. (I think the 1 in 0o100775 is the inode type, as in file, directory, symlink, etc., but I'm not sure). Regardless, it captures the entire unix mode so it works well on unix-like systems with newer backups.

[believeinlain]

Finally got it to pass CI on Windows.

[believeinlain]

Incorporated into pull request #194, which addresses all of issue #46