Set new min date on timeslider

[danrademacher]

In resolving OpenHistoricalMap/ohm-website#39 , we set the new datelimit to 1000 in this commit:
OpenHistoricalMap/ohm-website@6c2d044

Turns out we have stuff from Ancient Rome!

So we need to set an older min-date limit.

Over in https://github.com/OpenHistoricalMap/decimaldate-python, we include an example like this

Convert a ISO-formatted date to a decimal date, accounting for negative years (BCE) and for very large dates (-1000000-01-01).

Any reason not to set our floor at the beginning of recorded history?

@jimmyrocks was there a reason to pick 1000, like you tested other values and they failed? Or just seemed old enough?

[jeffreyameyer]

@jimmyrocks - any update here?

I need it to go waaaaay back. Thanks!

[danrademacher]

This is a change of a few characters, here:
OpenHistoricalMap/ohm-website@6c2d044

Make 1000 into say -5000, which we know is working on the standalone demo at -4000:
https://openhistoricalmap.github.io/openhistoricaltiles/leaflet-control-mbgltimeslider/demo/#14.600/40.80623/-73.91894/1920,-4000-2018

But we need to sort out Github repo access before we can change again

[danrademacher]

Currently -4000.