title: Beautiful Soup find all values for a given attribute, without specifying the tag
body: <p>Is there a way to get all values of a certain attribute?</p>
  
  <p>Example:</p>
  
  <pre><code>&lt;a title="title-in-a"&gt;
    &lt;b title="title-in-b"&gt; ... &lt;/b&gt;
    &lt;c title="title-in-c"&gt; ... &lt;/c&gt;
    &lt;d name="i-dont-care"&gt; ... &lt;/d&gt;
  &lt;/a&gt;
  </code></pre>
  
  <p>Can I get all titles, even if they are in different tags?</p>
  
  <p>Expected result:</p>
  
  <pre><code>['title-in-a', 'title-in-b', 'title-in-c']
  </code></pre>
  
  <p>To get all titles in <code>&lt;a&gt;</code>, I know I can do this:</p>
  
  <pre><code>for item in soup.find_all('a'):
      print item['title']
  </code></pre>
  
  <p>But how to do it for all tags, even without knowing the tags?</p>