Allow overriding default "next date"

[patcon]

Right now, this only works for the next upcoming event listed in spreadsheet date column. Would be cool if we could do https://civictechto.github.io/tweet-stub-helper?date=2019-04-02 to force a specific date. This would allow us to share a link for an event that's further away, without it being confusing why the hashtags are "wrong".