Reminder Plugin

**Is your feature request related to a problem? Please describe.**

It would be nice to have scheduled reminders sent.

**Describe the solution you'd like**

- Set a schedule, message text, and channel for one or more reminders
- See a list of set reminders
- Remove a specific reminder

**Describe alternatives you've considered**

Dedicated reminder bots exist, but require adding yet another bot

**Additional context**

Start by inspecting the MoDiBo repository's existing plugin and command architecture, then look for any scheduling or persistence support. Done means users can create one or more reminders with a schedule, message, and channel, list them, and remove a specific reminder.

Written by the indexing model from the issue text.