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)
```

kong.go

@@ -412,7 +412,7 @@ func (k *Kong) FatalIfErrorf(err error, args ...interface{}) {
 	}
 	msg := err.Error()
 	if len(args) > 0 {
-		msg = fmt.Sprintf(args[0].(string), args[1:]...) + ": " + err.Error() // nolint
+		msg = fmt.Sprintf(args[0].(string), args[1:]...) + ": " + err.Error() //nolint
 	}
 	// Maybe display usage information.
 	var parseErr *ParseError
@@ -439,11 +439,11 @@ func (k *Kong) LoadConfig(path string) (Resolver, error) {
 	if err != nil {
 		return nil, err
 	}
-	r, err := os.Open(path) // nolint: gas
+	r, err := os.Open(path) //nolint: gas
 	if err != nil {
 		return nil, err
 	}
-	defer r.Close() // nolint: gosec
+	defer r.Close()
 
 	return k.loader(r)
 }