title: How to add "target" attribute to `a` tag in ckeditor5?
body: <p>I have create my own plugin for link. Now I want to add some other attributes to <code>a</code> tag generated by the plugin, like <code>target</code>, <code>rel</code>.</p>
  
  <p>But I am not able to get it done. Here is the my plugins code for converter.
  What converters I should add so that <code>a</code> tag can support other attributes?</p>
  
   <pre class="lang-js prettyprint-override"><code>/**
   * @license Copyright (c) 2003-2018, CKSource - Frederico Knabben. All rights reserved.
   * For licensing, see LICENSE.md.
   */
  
  /**
   * @module link/linkediting
   */
  
  import LinkEditing from '@ckeditor/ckeditor5-link/src/linkediting';
  import {
      downcastAttributeToElement
  } from '@ckeditor/ckeditor5-engine/src/conversion/downcast-converters';
  import { upcastElementToAttribute } from '@ckeditor/ckeditor5-engine/src/conversion/upcast-converters';
  import LinkCommand from './uclinkcommand';
  import UnlinkCommand from './ucunlinkcommand';
  import { createLinkElement } from '@ckeditor/ckeditor5-link/src/utils';
  import { ensureSafeUrl } from './utils';
  import bindTwoStepCaretToAttribute from '@ckeditor/ckeditor5-engine/src/utils/bindtwostepcarettoattribute';
  
  /**
   * The link engine feature.
   *
   * It introduces the `linkHref="url"` attribute in the model which renders to the view as a `&lt;a href="url"&gt;` element.
   *
   * @extends module:core/plugin~Plugin
   */
  export default class UcLinkEditing extends LinkEditing {
      /**
       * @inheritDoc
       */
      init() {
          const editor = this.editor;
  
          // Allow link attribute on all inline nodes.
          editor.model.schema.extend( '$text', { allowAttributes: 'linkHref' } );
  
          editor.conversion.for( 'dataDowncast' )
              .add( downcastAttributeToElement( { model: 'linkHref', view: createLinkElement } ) );
  
          editor.conversion.for( 'editingDowncast' )
              .add( downcastAttributeToElement( { model: 'linkHref', view: ( href, writer ) =&gt; {
                  return createLinkElement( ensureSafeUrl( href ), writer );
              } } ) );
  
          editor.conversion.for( 'upcast' )
              .add( upcastElementToAttribute( {
                  view: {
                      name: 'a',
                      attribute: {
                          href: true
                      }
                  },
                  model: {
                      key: 'linkHref',
                      value: viewElement =&gt; viewElement.getAttribute( 'href' )
                  }
              } ) );
  
          // Create linking commands.
          editor.commands.add( 'ucLink', new LinkCommand( editor ) );
          editor.commands.add( 'ucUnlink', new UnlinkCommand( editor ) );
  
          // Enable two-step caret movement for `linkHref` attribute.
          bindTwoStepCaretToAttribute( editor.editing.view, editor.model, this, 'linkHref' );
  
          // Setup highlight over selected link.
          this._setupLinkHighlight();
      }
  }
  </code></pre>