Add !giveaway

[willow-bohlen]

(copied from this PR)

Giveaways in streams can be difficult when people are afk/don't want the item/are bots, so this command would streamline it to run the giveaway just once.

USAGE: !giveaway start time (Mod-Only) ------ Starts the giveaway countdown for a certain time, defaults to 60 !giveaway join -------------------------- Join the giveaway while it is counting down !giveaway end (Mod-Only) ------------ Stops the countdown and picks a winner. !giveaway cancel (Mod-Only) --------- Cancels the giveaway without picking a winner.