Add ability to define custom header element via callback

Hi Adam,

Thank you for a fantastic jQuery plug-in! I've been using it as part of an event management plug-in for WordPress. I needed a couple of drop-down filters added to the header of the calendar, so I've added an option: 'filters' which takes an object, with a filter ID as property, and a callback function as the value. 

The filters are added along with the buttons (if the button name matches a filter), and the callback defines the mark-up. So more generally - anything can be added to the header of the calendar (so 'filters' might be a misleading name).

Would be great to see (something like) this added!

**Example usage:**
(I've omitted the defining of the category terms)


    function category_dropdown(options){

         //options.categories stores category term objects
         var terms = options.categories;

         //Generate HTML
         var html="<select class='eo-cal-filter' id='eo-event-cat'>";
         html+="<option value=''>"+options.buttonText.cat+"</option>";
         for (var i=0;i<2; i++) {
              html+= "<option value='"+terms[i].slug+"'>"+terms[i].name+"</option>";
         }
         html+="</select>";

         return html;
    }

      $('#calendar').fullCalendar({
            filters:{
                  category:  category_dropdown,
            },
            header: {
                  left: 'category'
            },
      });