Added marcsnapshot command.

Status: Experimental.

marcsnapshot takes a list of MARC files and deduplicates records, keeping only the "newest", if multiple versions are found.

Field 001 is used as ID. Field 005 value is used to sort.

Example: three marcfiles, possibly with duplicates.
Field 001 values can be identified by the first 9 chars.

    $ marcsnapshot -verbose -l 9 2010.mrc 2011.mrc 2012.mrc > out.mrc