Use oldest param instead of limit/max_results #95
Description
Right now, we use a results_per_channel variable to set how many questions to fetch, and allow users to increase that limit 10 questions at a time. It would be more transparent to instead use the oldest parameter in the Slack API call, and display something like "Showing questions since {date}". If we get back as far as the user can see (no more questions are returned beyond a date that's more recent than the oldest date), we should update the display to show that date and not allow the user to try to go further back (that second part might be a separate enhancement to this first enhancement; make a new issue before closing this one if so).
Part of the reason this approach would be better: To get to the results_per_channel limit, some channels have to go way back, while a channel like help-r--general might only be back a few weeks with the same limit.