I got rendering working properly!

By implementing `start()` on the TabBarCoordinator, I can essentially instantiate the coordinator that I want. The TabBarCoordinator is still very incomplete, but I was able to get the TodayCoordinator working in a way that is promising.

The TabBarCoordinator shows TodayCoordinator from TabBarCoordinator.start for now. Then, TodayCoordinator.start calls show(route:) which is defined on the parent to just... show the view. I pass it the startRoute. I had to debug by printing out the error and learned that I had to make sure that TodayRoute wasn't returning nil as it isn't presenting a child coordinator.

Then, I was able to use TodayCoordinator.view(for:) to render two views. The first view represents the feed, the second view represents a detail. I was able to go back without the back button by making my own button that calls pop(animated:).