Reminders functionality

[jasonrdsouza]

It would be great if coffeebot supported reminders like slackbot.

This would be useful for mundane things like reminding people when it's coffee run time, and more scheduled things like reminding book club participants to finish the readings before meetings.

@channel reminders and timestamps is probably good enough for it to be useful, but bonus points if you can specify the people/ channel to remind, and if we can use human friendly datetimes (in 3 days, tomorrow, etc)