title: Block preview not visible when not in edit state
body: <p>I have started using Gutenberg custom blocks, and I have built custom blocks successfully!</p>
  
  <p>But I have an issue, I need to see the preview of the block when I am not editing it on the Admin side, I don't think Gutenberg provides it, because I searched the documentation, searched the internet but nothing found regarding preview of the block when not in edit state.</p>
  
  <p>I have used Block Lab Plugin and it do have a preview, if Gutenberg don't provide it, how can I have a preview? Any Trick or Hack?</p>
  
  <p>This is my <code>block.js:</code></p>
  
  <pre><code>(function(blocks, components, element) {
      var el = element.createElement;
      var registerBlockType = blocks.registerBlockType;
      var TextControl = components.TextControl;
  
      registerBlockType("blocks/test-block", {
        title: "Test Block",
        description: "A custom block.",
        icon: "businessman",
        category: "common",
        attributes: {
          headline: {
            type: "string"
          }
        },
  
        edit: function(props) {
          var headline = props.attributes.headline;
          var onHeadlineChange = function(val) {
            props.setAttributes({
              headline: val
            });
          };
  
          return el(TextControl, {
            value: headline,
            label: "Headline",
            onChange: onHeadlineChange
          });
        },
        save: function(props) {
          return el(
            "h3",
            {
              className: "headline"
            },
            props.attributes.headline
          );
        }
      });
    })(window.wp.blocks, window.wp.components, window.wp.element);
  </code></pre>