errgroup: add package

Package errgroup provides synchronization, error propagation, and
Context cancellation for groups of goroutines working on subtasks of a
common task.

Change-Id: Ic9e51f6f846124076bbff9d53b0f09dc7fc5f2f0
Reviewed-on: https://go-review.googlesource.com/24894
Reviewed-by: Sameer Ajmani <sameer@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>