Don't transfer files created after <date>

[rhine3]

Add option to not transfer files created on or after a particular year/month/day. Suggested behavior:

$ python sd-transfer.py -p MSD -l /Volumes/seagate3/field-data/20190306 -t 20190306

The line above would ideally only transfer files created before the given date, 2019-03-06. This would be applicable if recorders were brought in from the field on 2019-03-06 and we wanted to delete all data from the window of time when recorders were being picked up.

Implementing this in a cross-platform way could be tricky. It's easier to access the "last modified" date than the "first created" date so the "last modified" date could be an acceptable substitute for our purposes.