Implement remindme command

[mfirhas]

Command: /remindme

This command will remind you on the date or time you specify.

If you invoke the command in a reply, reminder will add context linking to it.

Time format:

• tomorrow: reminder set for tomorrow.
• nd: reminder set for n days starting tomorrow. E.g. 2d
• ny: reminder set for n years starting tomorrow. E.g. 1y (Hope the server will still be running)

Usecase(beside general reminder):

• You promise to reply someone in a group chat, but can't do it now.
• Someone make a bet
• You plan something, you can add their usernames and they get reminded too. E.g. watching movies together, playing dota together.
• You can use this to remind you to pay zakat, after calculating it in /zakat command.
• Etc.