title: Tinymce: allow userdefined tags
body: <p>Tinymce is removing unknown tags. I want to use userdefined tags, but I did not find the allowed Tags and their rules defined in Tinymce 4 to adjust it?
  I want insert my own tags with the function: tinyMCE.activeEditor.dom.create, so tiny adds and accept it like the mark-Tag:</p>
  
  <pre><code>&lt;mark&gt;Mytext&lt;/mark&gt;
  </code></pre>
  
  <p>so it should look like this </p>
  
  <pre><code>&lt;user-tag&gt;Mytext&lt;/user-tag&gt;
  </code></pre>