ScrollSpy should work with pushState urls instead of hashes

[devth]

I've added a simple implementation. Allows the use of data-target attributes in place of hash urls.

<a href="/scrollspy" data-target="#scrollspy">Scrollspy</a>

now works the same as

<a href="#scrollspy">Scrollspy</a>

It gives data-target priority over href when present, so href can point to anything.