title: Gutenberg: Allow additional formatting in RichText
body: <p>I have created a fairly simple accordion block, and it works great for basic text. The problem is that the control I am using for the accordion content is the RichText, which only allows for basic formatting such as bold.</p>
  
  <p>What if I wanted to create an Unordered List as well as basic text? I am currently using <code>multiline: "p"</code>, but how can I add additional elements so that I can also have UL elements in there as well?</p>
  
  <p>The only two ideas I can think of, I cannot figure out how to implement. The first is to extend the block toolbar with <code>BlockControls</code> to include additional formatters for UL, and the second is to use another element instead of RichText - such as Freeform (which might have been renamed to Classic Editor?) - but I cannot find any documentation on these.</p>
  
  <p>Here is an example of my current code:</p>
  
  <p>ATTRIBUTES</p>
  
  <pre><code>attributes: {
      title: {
          type: 'string',     
          selector: '.hd-accordion-title',
      },  
      content: {
          type: 'array',
          source: 'children',
          selector: '.hd-accordion-content',
      }
  },
  </code></pre>
  
  <p>EDIT</p>
  
  <pre><code>edit: function( props ) {
          var title = props.attributes.title;     
          var content = props.attributes.content;
  
          function onChangeTitle(newTitle) {
              props.setAttributes({
                  title: newTitle
              });
          }
  
          function onChangeContent(newContent) {
              props.setAttributes({
                  content: newContent
              });
          }   
  
          return [
              (
                  &lt;div className={"hd-accordion"}&gt;
                      &lt;RichText
                          tagName="h3"
                          className= "hd-accordion-title"
                          value= { title }
                          onChange= { onChangeTitle }
                          placeholder = "Title"
                          keepPlaceholderOnFocus = { true }
                          multiline= { false }
                      /&gt;              
                      &lt;RichText
                          tagName="div"
                          className="hd-accordion-content"
                          value={ content }
                          onChange= { onChangeContent }
                          placeholder = "content"
                          multiline="p"
                      /&gt;
                  &lt;/div&gt;
              )
          ];
      },
  </code></pre>