Add text-pitch-alignment, refs #459

[yhahn]

• Full writeup from @lucaswoj here: https://github.com/mapbox/mapbox-gl-style-spec/issues/459
• Implementation in progress here: https://github.com/mapbox/mapbox-gl-js/pull/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: https://github.com/mapbox/mapbox-gl-js/pull/2668/commits/e93f91e49f3370f0e2ecb43bffb49f5b6b66f08c
• Native implementation of inheritance: https://github.com/mapbox/mapbox-gl-native/pull/5288/commits/e91dba9910864ead78e881d21b717a07c5ccd5f0