code: function visitLink(ast, vFile, isSimple) {
    if (isSimple) {
      return visit(ast, 'link', (node, index, parent) => replaceLinkWithEmbedded(node, index, parent, vFile));
    }
  
    return visit(ast, 'link', node => replaceUrlWithGraph(node, vFile));
  }
docs: If links have a title attribute called `mermaid:`, follow the link and
  depending on `isSimple`, either generate and link to the graph, or simply
  wrap the graph contents in a div.
  
  @param {object}  ast
  @param {vFile}   vFile
  @param {boolean} isSimple
  @return {function}