Convert env management to a resolver.

This ensures that hooks correctly detect when values are set via
environment variables. Fixes #73.

model.go

@@ -3,7 +3,6 @@ package kong
 import (
 	"fmt"
 	"math"
-	"os"
 	"reflect"
 	"strconv"
 	"strings"
@@ -326,16 +325,6 @@ func (v *Value) ApplyDefault() error {
 // Does not include resolvers.
 func (v *Value) Reset() error {
 	v.Target.Set(reflect.Zero(v.Target.Type()))
-	if v.Tag.Env != "" {
-		envar := os.Getenv(v.Tag.Env)
-		if envar != "" {
-			err := v.Parse(ScanFromTokens(Token{Type: FlagValueToken, Value: envar}), v.Target)
-			if err != nil {
-				return fmt.Errorf("%s (from envar %s=%q)", err, v.Tag.Env, envar)
-			}
-			return nil
-		}
-	}
 	if v.Default != "" {
 		return v.Parse(ScanFromTokens(Token{Type: FlagValueToken, Value: v.Default}), v.Target)
 	}