Preserve scroll position when going back/forward

[ColemanGariety]

Similar to #190.

When using the traditional forward/back buttons, the scroll position is preserved. If you scroll down, click a link, and then go back, you won't end up back at the top of the page.

Because it emulates the forward/back buttons, Navi should also preserve scroll position when going forward and back.

[stnmonroe]

Hey, I'm also in need of this fix before having to rip apart an app to make certain features work. @JacksonGariety did you ever find a workaround for this?