Add a --date filter and some other quality-of-life improvements to the generated data

[nemec]

Hi, thanks for the useful tool! I had some small ideas for improvement that I've packed into four commits in case you find them useful, too.

1. The script requires pandas, but it was not listed in the pip commands in the README. I've added it to the docs.
2. I added a --date argument to the script to append a date filter to the --csv and --map URLs, which (when loaded in the browser) will only show posts older than the given date. It uses something similar to this algorithm (I can't find where I originally got the algorithm from) to calculate the proper ID number. To give a concrete use case, you could search for the location of the "Capitol Hill Autonomous Zone" in Seattle in early June 2020 during the George Floyd protests. This will bring up locations with photos of the Zone while it was "occupied", even though that date was many months in the past now.
3. I added a pink map marker to the map view that represents the original searched lat/lon so it's easier to orient yourself if you're browsing a map where you don't know the location well.
4. URLs on the map markers now always open in a new tab if you single-click on one instead of navigating away from the map page itself.

[loganwilliams]

These are great, thank you for the contribution.