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.
As @mojavelinux said` 'asciidoctor/extensions'` is needed when loading extensions from the cli.
So you need to add the following line:
```ruby
require 'asciidoctor/extensions' unless RUBY_ENGINE == 'opal'
```