Add Discord webhook support

[tempusthales]

Adds Discord webhook support mirroring the existing Slack integration.

A new discord block can be added to JSON, YAML, and plist configs:

discord:
  enabled: true
  webhook_url: https://discord.com/api/webhooks/your-webhook-id/your-webhook-token
  text: "Custom Discord message supporting Markdown."
  attachments:
    - json
    - plist
    - yaml
    - markdown
    - html

Changes:

• Added DiscordConfiguration.swift model mirroring SlackConfiguration.swift
• Added Discorder.swift helper mirroring Slacker.swift
• Updated Configuration.swift to parse the discord config block
• Updated Kmart.swift to call Discorder.send when Discord is enabled