Add support for reminders in Slack

[andrew-cybsafe]

It would be awesome if Slack reminders could be synced into taskwarrior.

[arsatiki]

This might be limited by Slack more than anything. I tried to write a plugin to fetch them some time ago... but there's no API to fetch the new reminders. The old API only fetches the ones created before the new API was released. Dunno if the situation has changed during the summer but I doubt it.