Support Start and End alignment keywords

[nicoburns]

Report

CSS defines both start/end and flex-start/flex-end alignment keywords. The difference is that the flex-* versions are affected by both direction and flex-direction whereas the other versions ignore flex-direction.

Yoga only has FlexStart and FlexEnd variants in it's Align and Justify enums, and this issue is a request for Start and End to be added (and also to check that Yoga's FlexStart and FlexEnd do actually correspond to flex-start and flex-end semantics).