[xattrs] '--xattrs' option that writes metadata to extended attributes (implemented as a postprocessor)

I think you guys are going to like this final version of my xattr writer.

I listened to your previous objections, and rewrote most of it. It's now a postprocessor (`XAttrMetadataPP`), and it will only load the python `pyxattr` module (or search the path for `setfattr`) if the postprocessor is enabled.

Plus, it can be enabled with the `--xattrs` option.