title: Extract title with rvest
body: <p>I got this xml_nodeset (I don't even know if that's the correct term) from a economic calendar table over at forexfactory.com:</p>
  
  <pre><code>&lt;td class="calendar__cell calendar__previous previous"&gt;44.7&lt;/td&gt;
  &lt;td class="calendar__cell calendar__previous previous"&gt;&lt;span class="revised worse" title="Revised From -0.6%"&gt;-1.1%&lt;span class="icon icon--revised"&gt;&lt;/span&gt;&lt;/span&gt;&lt;/td&gt;
  </code></pre>
  
  <p>In the first case I want to get an empty string or NA, in the second case the value "Revised From -0.6%".</p>
  
  <p>Basically, I want an extra column that is empty if there is no revised value and holds the revised value if there is any.</p>
  
  <p>I tried </p>
  
  <p><code>%&gt;% html_attr(x, "title")</code> and  <code>%&gt;% html_attrs(x)</code> inspired by this question <a href="https://stackoverflow.com/questions/28350833/scraping-image-titles-with-rvest">here</a> without success.</p>
  
  <p>Of course, x holds the xml_node.</p>
  
  <p>Sorry if it's a noob question!</p>