In the latest builds of Atom, this no longer works so I've unpublished it until such time that I can fix it. I highly recommend the excellent go-plus package as a replacement.
Runs goimports on file save.
goimports also executes go fmt, so you don't need to do both.
You must have goimports already installed for this to work.
$ go get code.google.com/p/go.tools/cmd/goimports
See goimports for more information.
You may have to set the full path to goimports in the settings.
$ which goimports
/Users/brianstarke/go/bin/goimports
Definite hat tip to lsegal's atom-go-format for inspiration, I used his project as a jumping off point.