Add a prop to overwrite default class

[jamesallan93]

I think that a prop for overwriting the default class on the <ul/> would be handy.
So using js to overwrite wouldn't be needed.

 let form = document.getElementById("form_wiz");
 form?.children[0].classList.add("nav", "nav-pills", "nav-fill");

<FormWizard ulClass="class-1 abc primary">
   _...some code_
</FormWizard>