title: FullCalendar Custom/Override Header title
body: <p>I want change the titleFormat of my calendar. </p>
  
  <p>At present, the title is in an h2 tag and we cannot personalize it by adding html code to the titleFormat option. 
  For example put a span tag on my fixed value of the titleFormat (which is [] there).</p>
  
  <p>I would want to know if it is possible to override the updateTitle method of the class Header, without modifying fullcalendar.js. Or another possibility.</p>
  
  <p>I have 2 views : week with title format '[Semaine] W' and custom one Day with title format 'dddd D MMMM YYYY'.</p>
  
  <p>My version of FullCalendar is v2.3.0.</p>