(#3) Adds Discord Notification Plugin

[JPRuskin]

Description

Adds a Discord notification plugin, similar to the Gitter implementation.

Motivation and Context

We use Discord as one of the methods of communicating with maintainers of chocolatey-au using packages. Having this hook will enable us to react more quickly to package failures, and be aware of successful updates.

What has been implemented

Has two options for posting, a simple text output identical to the default Gitter notification and a more complex Discord-specific card.

Examples of output:

• First is the simple output
• Second is the "no failures" card output
• Third is the "failures" card output

How has this been tested?

• Tested against a channel in Discord, but not during a full cAU run

The only thing I'm particularly unsure about is if Parameter(Mandatory) will throw a wobbler, or how AU calls these things. I intended to ask KN.

Types of Changes

• New feature (non-breaking change which adds functionality)

Fixes #3.