Add possibility to keep current scroll position in the Turbolinks.visit API

On certain occasions some might need to call `Turbolinks.visit()` manually.
In my case I call `Turbolinks.visit(location.href)` to reload the content of the current page -- this is an improvised replacement for `location.reload()`.
Problem is, that if you want to reload the current page content for, say, new elements created by AJAX form submission, the page will scroll back to top, yet you want to keep the scroll position.

Thus, I've added a second parameter to the `Turbolinks.visit()` method, which defaults to `true`.
Nice to know that in JavaScript `null != false`, so the default value initialisation works.