golangci-lint: Upgrade, fix issues (#397)

The golangci-lint being used was quite dated.
This change upgrades to the latest version.
Adds and updates exclusions based on new failures.

Note on revive:
I've included an opt-out for unused parameters for revive
because turning `newThing(required bool)` to `newThing(_ bool)`
is a loss of useful information.

The changes to the Go files are to fix the following issues:

```
camelcase.go:16: File is not `gofmt`-ed with `-s` (gofmt)
config_test.go:50:18: directive `//nolint: gosec` is unused for linter "gosec" (nolintlint)
defaults_test.go:28:25: G601: Implicit memory aliasing in for loop. (gosec)
doc.go:5: File is not `gofmt`-ed with `-s` (gofmt)
kong.go:446:18: directive `//nolint: gosec` is unused for linter "gosec" (nolintlint)
kong_test.go:503:20: G601: Implicit memory aliasing in for loop. (gosec)
model.go:493:10: composites: reflect.ValueError struct literal uses unkeyed fields (govet)
scanner.go:112: File is not `gofmt`-ed with `-s` (gofmt)
```

And to address broken nolint directives reported as follows by
golangci-lint.

```
[.. skipped .. ]
tag.go:65:1: directive `// nolint:gocyclo` should be written without leading space as `//nolint:gocyclo` (nolintlint)
tag.go:206:51: directive `// nolint: gocyclo` should be written without leading space as `//nolint: gocyclo` (nolintlint)
util_test.go:23:43: directive `// nolint: errcheck` should be written without leading space as `//nolint: errcheck` (nolintlint)
util_test.go:51:22: directive `// nolint: errcheck` should be written without leading space as `//nolint: errcheck` (nolintlint)
```

model.go

@@ -490,6 +490,9 @@ func reflectValueIsZero(v reflect.Value) bool {
 	default:
 		// This should never happens, but will act as a safeguard for
 		// later, as a default value doesn't makes sense here.
-		panic(&reflect.ValueError{"reflect.Value.IsZero", v.Kind()})
+		panic(&reflect.ValueError{
+			Method: "reflect.Value.IsZero",
+			Kind:   v.Kind(),
+		})
 	}
 }