Allow users to set the header

I am introducing two options ms-header and ms-selected so that you can set what is displayed when no or multiple options are selected.  ms-selected = "There are {{myModel.length}} options selected" would be an example of an expression you can use to set the header.
updating example
Update index.html
Merge pull request #25 from runxc1/patch-1

Allow users to set the header