Make the onPreview() callback asynchronous.

This makes it possible to perform asynchronous operations while in the
onPreview() callback, for instance using an AJAX request:

    $("#form_content").markdown({
      onPreview: function(editor, callback) {
        jQuery.ajax({
          type: "POST",
          url: "/render-markdown",
          data: { content: editor.$textarea.val() },
          success: callback
        });
      }
    });

A default implementation of onPreview() is provided, using an updated
parseContent() method which doesn't call the onPreview() callback
anymore.

This effectively adds a parameter to the onPreview() callback, but
backward compatibility is retained by checking the onPreview() callback
return value and using it if it exists.