Add option to support timing

[nrgetik]

The ability to constrain archived Snaps not only by location but also by time would be useful. Just to illustrate the idea:

snapmap-archiver -o . -l='LAT,LONG' -r 5000 -t 5d  --write-json

snapmap-archiver -o . -l='LAT,LONG' -r 5000 -t 36h --write-json

The new option being -t which accepts some concept of time in either hours or days (minutes seems unneccesasry, but 🤷‍♂️)