flexmark-java extension for emoji shortcuts

emoji shortcut conversion: 

* [Emoji Cheat Sheet] shortcuts 
* [GitHub Emoji API] shortcuts
* any of the above shortcuts to [Unicode Emoji List, v11.0] Unicode Characters.

[`emoji-cheat-sheet.com/public/graphics/emojis/`]: https://github.com/WebpageFX/emoji-cheat-sheet.com/tree/master/public/graphics/emojis
[Emoji Cheat Sheet]: https://github.com/WebpageFX/emoji-cheat-sheet.com
[GitHub Emoji API]: https://api.github.com/emojis
[Unicode Emoji List, v11.0]: https://unicode.org/emoji/charts/emoji-list.html
