Read an external file, and dump entries to the
dumpfile according to the list of entry names contained in the external file.
The external file must be in comma delimited format: each line should contain
either the entry name only, or a series of fields, separated by semicolons,
the last of which is the desired entry name. The entries are dumped to the
dump file in BibTEX format, irrespective of the dump mode in force.