Refactor voice controllers

[frederoni]

Fixes #280

Split up voice controllers into RouteVoiceController and PollyVoiceController.

Replace AVSpeech with Polly:

navigationViewController.voiceController = PollyVoiceController(identityPoolId: "…")

@1ec5 @bsudekum 👀

[1ec5]

With this refactoring, we can modify the podspec to split PollyVoiceController.swift into a subspec: #280 (comment).

We could further streamline the setup process: PollyVoiceController.swift could define a category method on NavigationViewController that sets up the voice controller. That would come at the cost of modularity, I suppose.

[1ec5]

shouldSpeak(for:)

[1ec5]

This is a very vague name. Could we fold this into alertLevelDidChange(_:) and allow subclasses to override that method?

[frederoni]

Interesting idea about the category but less obvious to grasp and debug.

We should add a new target for PollyVoiceController when we create the podspec to
provide modularity for both Carthage and CocoaPods users.

[1ec5]

If we add a separate target, we'd have to add a separate podspec. Subspecs can't coexist with separate modules.