Fixed problem with `toggles` containing a URL before the fragment identifier

Sometimes it is necessary to add an URL before the fragment identifier of the toggles like this:
<span>**ht<span>tp://w</span>ww.example.org/foo.html**</span>#bar or <span>**foo.html**</span>#bar

This caused an error at line [194](https://github.com/cferdinandi/smooth-scroll/blob/master/src/js/smooth-scroll.js#L194):
````
Uncaught SyntaxError: Failed to execute 'querySelector' on 'Document':
'http://www.example.org/foo.html#bar' is not a valid selector.
````
Using `toggle.hash` instead of `toggle.getAttribute('href')` in line 284 ensures that only the fragment identifier (#bar) will be passed to `animateScroll()`.