Discord Support

[Just-Insane]

Per the discussion here: prometheus/alertmanager#1365 it would be nice to add support for Discord.

To start, it would be nice to have Discord notifications of alerts, commands can be added later.

I've never worked with Go before, but it looks like everything should be self-contained, I would just need to add a pkg/Discord folder like there is a pkg/telegram folder, with the Discord bot configuration?

It looks like cmd/alertmanager-bot/main.go might need to be updated as well.