Added discord notifier

[Niller2005]

No description provided.

[mdominoni]

When executing, showing this error.

zenbot/conf-sample.js:189
c.notifiers.discord.webhook.id = 'YOUR-WEBHOOK-ID'
^

TypeError: Cannot set property 'id' of undefined
at Object. (/opt/zenbot/conf-sample.js:189:32)
at Module._compile (module.js:570:32)
at Object.Module._extensions..js (module.js:579:10)
at Module.load (module.js:487:32)
at tryModuleLoad (module.js:446:12)
at Function.Module._load (module.js:438:3)
at Module.require (module.js:497:17)
at require (internal/module.js:20:19)
at module.exports (/opt/zenbot/boot.js:12:18)
at Object. (/opt/zenbot/zenbot.js:20:1)

[mdominoni]

Solved by upgrading to node v8.6.0