Add text-pitch-alignment, refs #459

[yhahn]

• Full writeup from @lucaswoj here: Expand support for "*-pitch-alignment" and "*-size-alignment" #459
• Implementation in progress here: text-pitch-alignment mapbox-gl-js#2668

[lucaswoj]

LGTM 🚢

[yhahn]

@lucaswoj @jfirebaugh per our discussion yesterday,

• Removed the default value for text-pitch-alignment
• Updated the description to describe how the default does text-rotation-alignment => text-pitch-alignment

I think the "don't lie in the spec" approach is a good place to start and figuring out a more robust way to specify these edge cases in the spec a quest we can take on in the future.

• JS implementation of inheritance: mapbox/mapbox-gl-js@e93f91e
• Native implementation of inheritance: mapbox/mapbox-gl-native@e91dba9