Basic stickers support

[edshot99]

Summary

Include sticker objects if stickers were sent in a message. Fixes #18

This could be expanded to let the bot access server sticker's, but I just wanted something basic at the moment.

Example

  event.message.stickers.each do |sticker|
    file_url = "https://cdn.discordapp.com/stickers/#{sticker.id}.#{sticker.extension}"
    populate_cdn_sticker("", file_url)
  end

---

Added

• Sticker data type.

Changed

• Message data type to include :sticker_items and a :stickers alias.

[Droid00000]

Thanks for the PR! Everything looks fine to me besides the comments