Enable gofumpt linter; format code gofumpt -w .

[alexandear]

This PR:

• Formats all code with the command gofumpt -w . (removed empty lines, replaces var a = with a :=, } indentation, octal literals etc.)
• Enables gofumpt linter in golangci-lint args to ensure everyone will follow the consistent style in future PRs;
• Removes gofmt because it's rules are a subset of gofumpt.

[erikdubbelboer]

Thanks! I like it!