Add --since option to marc_export cl script

Add --since option to marc_export cl script

It's useful to be able to export records from a certain
creation or edit date.
1) We have workflows that require periodic record exports
2) Makes testing quick and easy

Only works with the --all flag

Example: marc_export --all --since=$(date -d '2 months ago' +%Y-%m-%d)
Example: marc_export --all --edited --since=$(date -d '2 days ago' +%Y-%m-%d)

Signed-off-by: Mark Cooper <markchristophercooper@gmail.com>
Signed-off-by: Dan Scott <dscott@laurentian.ca>