[added] ScrollsOnTransition mixing

When a page’s content is long and you transition
to a new route, the scroll position persists
and its super dumb.

This mixin goes on a route handler that you know
has long content and scrolls the window up when
you leave the page.

Not a bad idea to put this on every route handler

closes #121
I initially put this on willTransitionTo, but the entire issue arises when the previous page has long contents, so it seems to make sense to do it on the page that causes the problem you want to mitigate.