Turn-by-turn voice navigation

[chrisduerr]

Reading navigation instructions while driving is generally not optimal, so it would be much better if the instructions could be given using text-to-speech. Valhalla even returns special voice instruction text from its API to help with this.

The main issue with this is the potential complexity sound and TTS could have on the project. It might not be worthwhile to add this if its complexity would dwarf everything else. However if there's a minimal way to get this added, I think it would be a big bonus.