titleView prop in Navigator.Config?

Would it make sense to add a `titleView` prop to `Navigator.Config` to be able to use a custom view in place of the title?
Something like:
```

```
where `SomeView` is
```
...
AppRegistry.registerComponent('SomeView', () => SomeView)
```
and possibly a `titleViewProps` prop for props to pass to the view.

It's not the ideal API, I think, I'd want this instead
```
}
/>
```
but I'm not sure how feasible that is

Super useful either way IMO, what do you guys think?

(EDIT: this might make sense only for iOS, not sure about that)

Start by reviewing the existing Navigator.Config API and how title configuration is handled, with particular attention to the possible iOS-only scope mentioned in the issue. Determine whether a custom view and props can be supported, then define completion as Navigator.Config rendering that view in place of the title with the intended props behavior.

Written by the indexing model from the issue text.