Add Reminders to SaltBot

Created by: HighSaltLevels

What/Why

It would be really nice if SaltBot can keep up with reminders. This PR adds functionality to do just that.

Setting reminders

Setting reminders must follow the format !remind set <REMINDER-MSG> in <NUMBER> <UNIT>.

For Example:

!remind set do something in 5 seconds

Showing reminders

To show all reminders, use !remind show and the reminders will output in this way:

3567fca7: "eat pizza" at Feb 13, 2021 at 07:52:14 PM
ad90d4a6: "eat another pizza" at Feb 13, 2021 at 07:52:35 PM
d125250d: "eat other pizza" at Feb 13, 2021 at 07:51:21 PM

Deleting reminders

You can delete reminders by their ID. If you wanted to delete the first reminder in the list above, use:

!remind delete 3567fca7

Future enhancements to add:

• Allow users to specify an end date instead of a timespan (something like ends Friday November 2nd at 5:00 PM)
• Sort the list of reminders in order of ending soonest (consider allowing user to specify sorting)