Introduced the Markdown plugin.

src/markdown.js

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+/**
+ * @license Copyright (c) 2003-2019, CKSource - Frederico Knabben. All rights reserved.
+ * For licensing, see LICENSE.md or https://ckeditor.com/legal/ckeditor-oss-license
+ */
+
+/**
+ * @module markdown-gfm/markdown
+ */
+
+import Plugin from '@ckeditor/ckeditor5-core/src/plugin';
+import GFMDataProcessor from './gfmdataprocessor';
+
+/**
+ * The GitHub Flavored Markdown (GFM) plugin.
+ *
+ * For a detailed overview, check the {@glink features/markdown Markdown feature documentation}.
+ *
+ * @extends module:core/plugin~Plugin
+ */
+export default class Markdown extends Plugin {
+	/**
+	 * @inheritDoc
+	 */
+	constructor( editor ) {
+		super( editor );
+		editor.data.processor = new GFMDataProcessor();
+	}
+
+	/**
+	 * @inheritDoc
+	 */
+	static get pluginName() {
+		return 'Markdown';
+	}
+}

tests/markdown.js

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+/**
+ * @license Copyright (c) 2003-2019, CKSource - Frederico Knabben. All rights reserved.
+ * For licensing, see LICENSE.md or https://ckeditor.com/legal/ckeditor-oss-license
+ */
+
+import Markdown from '../src/markdown';
+import GFMDataProcessor from '../src/gfmdataprocessor';
+import ClassicTestEditor from '@ckeditor/ckeditor5-core/tests/_utils/classictesteditor';
+
+describe( 'Markdown', () => {
+	it( 'has proper name', () => {
+		expect( Markdown.pluginName ).to.equal( 'Markdown' );
+	} );
+
+	it( 'should set editor.data.processor', () => {
+		return ClassicTestEditor
+			.create( '', {
+				plugins: [ Markdown ]
+			} )
+			.then( editor => {
+				expect( editor.data.processor ).to.be.an.instanceof( GFMDataProcessor );
+			} );
+	} );
+} );