Add fixer for golangci-lint

[davidjb]

Name: golangci-lint
URL: https://github.com/golangci/golangci-lint

This was previously reported at #2864:

GoLangCI-lint has a fixer now: golangci-lint run --fix ./...

The ALE fixer will have the same issues we're having with the ALE linter.

This original issue was to add a fixer for golangci-lint and there may have been some confusion as the issue was closed on account of golangci-lint as a linter already existing.

As best I can tell and from what I've tried, golangci-lint isn't available as a fixer (e.g. it's not visible in :ALEFixSuggest nor has a file listed at https://github.com/dense-analysis/ale/tree/master/autoload/ale/fixers or visible elsewhere in the codebase).

[sigmavirus24]

@w0rp if there are some docs on how to contribute this easily, I'd be happy to give it a shot. I also think I'm seeing that there's no way to configure goimports with a local prefix.

[sigmavirus24]

I think #4853 fixes this. It's my first vim plugin contribution but as far as I can tell, it's passing tests.