Add support for Slack emoji aliases

[davidcelis]

Slack lets you add custom emoji as aliases to other emoji. When requesting a workspace's custom emoji from the emoji.list API endpoint (which is a convenient way to populate the original_emoji_set option), emoji aliases follow a pattern:

{
  "ok": true,
  "emoji": {
    "simple_smile": "https://a.slack-edge.com/80588/img/emoji_2017_12_06/apple/simple_smile.png",
    "octocat": "https://emoji.slack-edge.com/T02F9SUA63B/octocat/85961c43d7.png",
    "github": "alias:octocat",
    "happy":"alias:smile"
  },
  "cache_ts": "1635536920.033500"
}

You can add emoji as an alias to both other custom emoji, or even the stock unicode emoji. This patch supports both ✨

[davidcelis]

Oops, gonna close this out in favor of a new PR, sorry! I had another contribution I wanted to make so I'm going to use a feature branch instead of master 😅