Use goimports on save if available

[magisterquis]

Since goimports is a superset of gofmt functionality, (perhaps optionally) using it instead of gofmt on save not only formats code, but handles imports as well.

[parkr]

It may be helpful to know that simply putting goimports into TM_GOFMT doesn't work – goimports expects input from stdin and that's not how it works at the moment with gofmt in this bundle.