caltechlibrary/eprints2bags: Version 1.6.0

The following are the main changes in this release: * This release adds a new command-line option `--lastmod` (`-l` for short) that allows the user to specify a date/time stamp, to return only those records whose last-modified date/time stamp is no older than the given description. Valid descriptors are those accepted by the Python dateparser library. Example: `eprints2bags --lastmod "yesterday at noon" -a ...`. * This release also fixes a bug in parsing the values handed to `--id-list`. The file [CHANGES](CHANGES.md) contains a more complete change log that includes information about previous releases.