How to: metadata from xattrs
unix extended file attributes  can be indexed into the db and made searchable;

* `--db-xattr user.foo,user.bar` will index the xattrs `user.foo` and `user.bar`,
* `--db-xattr user.foo=foo,user.bar=bar` will index them with the names `foo` and `bar`,
* `--db-xattr ~~user.foo,user.bar` will index everything *except* `user.foo` and `user.bar`,
* `--db-xattr ~~` will index everything

however note that the tags must also be enabled with `-mte` so here are some complete examples:
* `-e2ts --db-xattr user.foo,user.bar -mte +user.foo,user.bar`
* `-e2ts --db-xattr user.foo=foo,user.bar=bar -mte +foo,bar`