D8 - Using Composer to download git repo
As per @igorw's answer  

{
        "repositories": [
            {
                "url": "https://github.com/w8tcha/CKEditor-CodeMirror-Plugin/blob/master/composer.json",
                "type": "git"
            }
        ],
        "require": {
            "ckeditor-codemirror-plugin": "~1.17.2"
        }

}


Then type


  composer require "ckeditor-codemirror-plugin":"1.17.2"