Clean up disparity between Context and Kong.

Previously, there was a confusing mix of functionality shared between
the two wherein you would need to use the Kong type for printing errors,
etc. but it did not have access to the context in order to print
context-sensitive usage information. This has been fixed.

Additionally, there are now fuzzy correction suggestions for flags and
commands

Also added a server example which shows how Kong can be used for parsing
in interactive shells. Run with:

    $ go run ./_examples/server/*.go

Then interact with:

    $ ssh -p 6740 127.0.0.1

kong.go

@@ -42,13 +42,15 @@ type Kong struct {
 	Stdout io.Writer
 	Stderr io.Writer
 
-	before        map[reflect.Value]HookFunc
-	resolvers     []ResolverFunc
-	registry      *Registry
+	before    map[reflect.Value]HookFunc
+	resolvers []ResolverFunc
+	registry  *Registry
+
 	noDefaultHelp bool
 	usageOnError  bool
 	help          HelpPrinter
-	helpOptions   HelpPrinterOptions
+	helpOptions   HelpOptions
+	helpFlag      *Flag
 
 	// Set temporarily by Options. These are applied after build().
 	postBuildOptions []Option
@@ -83,6 +85,7 @@ func New(grammar interface{}, options ...Option) (*Kong, error) {
 	}
 	model.Name = filepath.Base(os.Args[0])
 	k.Model = model
+	k.Model.HelpFlag = k.helpFlag
 
 	for _, option := range k.postBuildOptions {
 		if err := option(k); err != nil {
@@ -121,6 +124,7 @@ func (k *Kong) extraFlags() []*Flag {
 		k.Exit(1)
 		return nil
 	})
+	k.helpFlag = helpFlag
 	_ = hook(k)
 	return []*Flag{helpFlag}
 }
@@ -182,8 +186,15 @@ func (k *Kong) applyHooks(ctx *Context) error {
 	return nil
 }
 
-func formatMultilineMessage(w io.Writer, leader string, format string, args ...interface{}) {
+func formatMultilineMessage(w io.Writer, leaders []string, format string, args ...interface{}) {
 	lines := strings.Split(fmt.Sprintf(format, args...), "\n")
+	leader := ""
+	for _, l := range leaders {
+		if l == "" {
+			continue
+		}
+		leader += l + ": "
+	}
 	fmt.Fprintf(w, "%s%s\n", leader, lines[0])
 	for _, line := range lines[1:] {
 		fmt.Fprintf(w, "%*s%s\n", len(leader), " ", line)
@@ -192,16 +203,22 @@ func formatMultilineMessage(w io.Writer, leader string, format string, args ...i
 
 // Printf writes a message to Kong.Stdout with the application name prefixed.
 func (k *Kong) Printf(format string, args ...interface{}) *Kong {
-	formatMultilineMessage(k.Stdout, k.Model.Name+": ", format, args...)
+	formatMultilineMessage(k.Stdout, []string{k.Model.Name}, format, args...)
 	return k
 }
 
 // Errorf writes a message to Kong.Stderr with the application name prefixed.
 func (k *Kong) Errorf(format string, args ...interface{}) *Kong {
-	formatMultilineMessage(k.Stderr, k.Model.Name+": error: ", format, args...)
+	formatMultilineMessage(k.Stderr, []string{k.Model.Name, "error"}, format, args...)
 	return k
 }
 
+// Fatalf writes a message to Kong.Stderr with the application name prefixed then exits with a non-zero status.
+func (k *Kong) Fatalf(format string, args ...interface{}) {
+	k.Errorf(format, args...)
+	k.Exit(1)
+}
+
 // FatalIfErrorf terminates with an error message if err != nil.
 func (k *Kong) FatalIfErrorf(err error, args ...interface{}) {
 	if err == nil {