Add a default-enabled EnvResolver for the env tag.

kong.go

@@ -52,12 +52,13 @@ type Kong struct {
 // See the README (https://github.com/alecthomas/kong) for usage instructions.
 func New(grammar interface{}, options ...Option) (*Kong, error) {
 	k := &Kong{
-		Exit:     os.Exit,
-		Stdout:   os.Stdout,
-		Stderr:   os.Stderr,
-		before:   map[reflect.Value]HookFunc{},
-		registry: NewRegistry().RegisterDefaults(),
-		help:     PrintHelp,
+		Exit:      os.Exit,
+		Stdout:    os.Stdout,
+		Stderr:    os.Stderr,
+		before:    map[reflect.Value]HookFunc{},
+		registry:  NewRegistry().RegisterDefaults(),
+		help:      PrintHelp,
+		resolvers: []ResolverFunc{EnvResolver()},
 	}
 
 	for _, option := range options {