It comes up often enough that it's time to provide
the utility of a standard package.

R=r, mirtchovski, adg, rsc, n13m3y3r, ality, go.peter.90, lstoakes, iant, jan.mercl, bsiegert, robert.hencke, rogpeppe, befelemepeseveze, kevlar
CC=golang-dev
https://golang.org/cl/4557047