Refactor TakesArgs to use an interface for arg validation.

Fix some typos in README and comments.
Move arg validation to after flag validation so that the help flag is run first.
Pass the same args to ValidateArgs as the Run methods receive.
Update README.

Signed-off-by: Daniel Nephin <dnephin@gmail.com>

cobra_test.go

@@ -36,6 +36,7 @@ var cmdHidden = &Command{
 
 var cmdPrint = &Command{
 	Use:   "print [string to print]",
+	Args:  MinimumNArgs(1),
 	Short: "Print anything to the screen",
 	Long:  `an absolutely utterly useless command for testing.`,
 	Run: func(cmd *Command, args []string) {
@@ -75,7 +76,7 @@ var cmdDeprecated = &Command{
 	Deprecated: "Please use echo instead",
 	Run: func(cmd *Command, args []string) {
 	},
-	TakesArgs: None,
+	Args: NoArgs,
 }
 
 var cmdTimes = &Command{
@@ -89,7 +90,7 @@ var cmdTimes = &Command{
 	Run: func(cmd *Command, args []string) {
 		tt = args
 	},
-	TakesArgs: ValidOnly,
+	Args:      OnlyValidArgs,
 	ValidArgs: []string{"one", "two", "three", "four"},
 }
 
@@ -103,10 +104,9 @@ var cmdRootNoRun = &Command{
 }
 
 var cmdRootSameName = &Command{
-	Use:       "print",
-	Short:     "Root with the same name as a subcommand",
-	Long:      "The root description for help",
-	TakesArgs: None,
+	Use:   "print",
+	Short: "Root with the same name as a subcommand",
+	Long:  "The root description for help",
 }
 
 var cmdRootTakesArgs = &Command{
@@ -116,7 +116,7 @@ var cmdRootTakesArgs = &Command{
 	Run: func(cmd *Command, args []string) {
 		tr = args
 	},
-	TakesArgs: Arbitrary,
+	Args: ArbitraryArgs,
 }
 
 var cmdRootWithRun = &Command{
@@ -477,6 +477,10 @@ func TestRootTakesNoArgs(t *testing.T) {
 	c.AddCommand(cmdPrint, cmdEcho)
 	result := simpleTester(c, "illegal")
 
+	if result.Error == nil {
+		t.Fatal("Expected an error")
+	}
+
 	expectedError := `unknown command "illegal" for "print"`
 	if !strings.Contains(result.Error.Error(), expectedError) {
 		t.Errorf("exptected %v, got %v", expectedError, result.Error.Error())
@@ -493,7 +497,11 @@ func TestRootTakesArgs(t *testing.T) {
 }
 
 func TestSubCmdTakesNoArgs(t *testing.T) {
-	result := fullSetupTest("deprecated illegal")
+	result := fullSetupTest("deprecated", "illegal")
+
+	if result.Error == nil {
+		t.Fatal("Expected an error")
+	}
 
 	expectedError := `unknown command "illegal" for "cobra-test deprecated"`
 	if !strings.Contains(result.Error.Error(), expectedError) {
@@ -502,14 +510,18 @@ func TestSubCmdTakesNoArgs(t *testing.T) {
 }
 
 func TestSubCmdTakesArgs(t *testing.T) {
-	noRRSetupTest("echo times one two")
+	noRRSetupTest("echo", "times", "one", "two")
 	if strings.Join(tt, " ") != "one two" {
 		t.Error("Command didn't parse correctly")
 	}
 }
 
 func TestCmdOnlyValidArgs(t *testing.T) {
-	result := noRRSetupTest("echo times one two five")
+	result := noRRSetupTest("echo", "times", "one", "two", "five")
+
+	if result.Error == nil {
+		t.Fatal("Expected an error")
+	}
 
 	expectedError := `invalid argument "five"`
 	if !strings.Contains(result.Error.Error(), expectedError) {