Implement flag "resolvers". (#24)

* Propagate errors.
* Use junit test output.
* Expand role of DecodeContext to include Scanner.
* Inject resolved flags as Path elements in the Context.
  This allows all existing logic to apply seamlessly: hooks, required
flags, etc.
* Clarify that hooks can be called multiple times.

tag_test.go

@@ -65,6 +65,7 @@ func TestEscapedQuote(t *testing.T) {
 }
 
 func TestBareTags(t *testing.T) {
+	// nolint: govet
 	var cli struct {
 		Cmd struct {
 			Arg  string `arg`
@@ -80,6 +81,7 @@ func TestBareTags(t *testing.T) {
 }
 
 func TestBareTagsWithJsonTag(t *testing.T) {
+	// nolint: govet
 	var cli struct {
 		Cmd struct {
 			Arg  string `json:"-" optional arg`
@@ -95,6 +97,7 @@ func TestBareTagsWithJsonTag(t *testing.T) {
 }
 
 func TestManySeps(t *testing.T) {
+	// nolint: govet
 	var cli struct {
 		Arg string `arg    optional    default:"hi"`
 	}