feat(ngAnimate): conditionally allow child animations to run in parallel with parent animations

By default ngAnimate prevents child animations from running when a parent is performing an animation.
However there are a cases when an application should allow all child animations to run without blocking
each other. By placing the `ng-animate-children` flag in the template, this effect can now be put to
use within the template.

Closes #7946
feat($animate): allow directives to cancel animation events

Closes #7722