Adds a new inline extension about emoji icons

Adds a new inline extension about emoji icons so emoji icons can be used in any AsciiDoc document.
Yes you can do it with docinfo file but then in each project where you want to use it you need to copy it inside (and change the prefix accordantly) but with the extension you can download all your extensions in directory and use it in all your documents. Moreover when we can register extensions inside document, it would be even more useful.