Use gofumpt instead of goimports

[Jacalz]

I have been using https://github.com/mvdan/gofumpt recently and I think it is doing a great job with the formatting of code. I would suggest switching to using that instead.

[bjoerndemeyer]

Gofumpt is not go fmt but a particular style. It is not usable for all Go projects. But perhaps the go formatter should be made configurable.