add -s to goimports (as available in gofmt)

[rochaporto]

tracked here for simplicity, but the issue is in the vim usage (as goimports gets called on save).

issue is that goimports, which mainly adds import management to gofmt (and wraps the rest), is not exposing the -s flag:
https://golang.org/cmd/gofmt/

looking around code seems to be here:
https://github.com/golang/tools/blob/master/cmd/goimports/goimports.go

should be trivial to add.

discovered while trying this out.
http://goreportcard.com/report/rochaporto/ezgliding