[web-animations-1] Add Animation.compositeBefore/compositeAfter

[birtles]

From @birtles on September 30, 2015 4:29

Discussion here: https://lists.w3.org/Archives/Public/public-fx/2015JulSep/0028.html

Copied from original issue: w3c/web-animations#110

[birtles]

This is not going to happen in level 1. Moving to level 2.