#40 Support for extended attributes

[i30817]

I'd like the filesystem of squashfs to have extended attribute support.

Specifically creating with, and reading always.

Even more specifically, if you want to implement it outside of the xattr design, i'd like to get a amortized record of files crc32, md5, sha1 or similar, so a switch to add one of the common hashcode algorithms to 'id' files so external apps don't have to redo the crc32 would be great.