Add route overview button

[brsbl-zz]

We should add a route overview button to the navigation UI. Zooming out to see an overview of the full route and then zooming back in is a very common action when using turn-by-turn navigation. Adding this feature makes it a lot safer for a user to do this while driving.

Here's what it looks like on iOS:

cc @mapbox/navigation-android

[danesfeder]

Thanks @brsbl!

@cjballard As far as UI / UX for this, do you like the idea of adding a third button like iOS has in this screenshot? Or would you rather do something like Gmaps:

[Cjballard-zz]

@danesfeder @brsbl Here are two versions of that mocked up using the current route preview icon - the first, integrating the preview button more with the trip tray (more like Google) and introducing a third action button.

I think I prefer having it integrated into the trip tray, as it saves some map space and feels more where a user would expect to preview the current route they're on. Thoughts?