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

help.go

@@ -13,38 +13,43 @@ const (
 	defaultColumnPadding = 4
 )
 
-// HelpPrinterOptions for HelpPrinters.
-type HelpPrinterOptions struct {
+// HelpOptions for HelpPrinters.
+type HelpOptions struct {
+	// Don't print top-level usage summary.
+	NoAppSummary bool
+
 	// Write a one-line summary of the context.
 	Summary bool
 
-	// Write help in a more compact form, but still fully-specified.
+	// Write help in a more compact, but still fully-specified, form.
 	Compact bool
 }
 
 // HelpPrinter is used to print context-sensitive help.
-type HelpPrinter func(options HelpPrinterOptions, ctx *Context) error
+type HelpPrinter func(options HelpOptions, ctx *Context) error
 
 // DefaultHelpPrinter is the default HelpPrinter.
-func DefaultHelpPrinter(options HelpPrinterOptions, ctx *Context) error {
+func DefaultHelpPrinter(options HelpOptions, ctx *Context) error {
 	if ctx.Empty() {
 		options.Summary = false
 	}
 	w := newHelpWriter(ctx, options)
 	selected := ctx.Selected()
 	if selected == nil {
-		printApp(w, ctx.App.Model)
+		printApp(w, ctx.Model)
 	} else {
-		printCommand(w, ctx.App.Model, selected)
+		printCommand(w, ctx.Model, selected)
 	}
-	return w.Write(ctx.App.Stdout)
+	return w.Write(ctx.Stdout)
 }
 
 func printApp(w *helpWriter, app *Application) {
-	w.Printf("Usage:  %s", app.Summary())
-	printNodeDetail(w, &app.Node)
-	cmds := app.Leaves()
-	if len(cmds) > 0 {
+	if !w.NoAppSummary {
+		w.Printf("Usage: %s%s", app.Name, app.Summary())
+	}
+	printNodeDetail(w, app.Node)
+	cmds := app.Leaves(true)
+	if len(cmds) > 0 && app.HelpFlag != nil {
 		w.Print("")
 		if w.Summary {
 			w.Printf(`Run "%s --help" for more information.`, app.Name)
@@ -55,11 +60,12 @@ func printApp(w *helpWriter, app *Application) {
 }
 
 func printCommand(w *helpWriter, app *Application, cmd *Command) {
-	w.Printf("Usage:  %s %s", app.Name, cmd.Summary())
+	if !w.NoAppSummary {
+		w.Printf("Usage: %s %s", app.Name, cmd.Summary())
+	}
 	printNodeDetail(w, cmd)
-	if w.Summary {
-		w.Print("")
-		w.Printf(`Run "%s %s --help" for more information.`, app.Name, cmd.Path())
+	if w.Summary && app.HelpFlag != nil {
+		w.Printf(`Run "%s %s --help" for more information.`, app.Name, cmd.FullPath())
 	}
 }
 
@@ -76,12 +82,12 @@ func printNodeDetail(w *helpWriter, node *Node) {
 		w.Print("Arguments:")
 		writePositionals(w.Indent(), node.Positional)
 	}
-	if flags := node.AllFlags(); len(flags) > 0 {
+	if flags := node.AllFlags(true); len(flags) > 0 {
 		w.Print("")
 		w.Print("Flags:")
 		writeFlags(w.Indent(), flags)
 	}
-	cmds := node.Leaves()
+	cmds := node.Leaves(true)
 	if len(cmds) > 0 {
 		w.Print("")
 		w.Print("Commands:")
@@ -114,16 +120,16 @@ type helpWriter struct {
 	indent string
 	width  int
 	lines  *[]string
-	HelpPrinterOptions
+	HelpOptions
 }
 
-func newHelpWriter(ctx *Context, options HelpPrinterOptions) *helpWriter {
+func newHelpWriter(ctx *Context, options HelpOptions) *helpWriter {
 	lines := []string{}
 	w := &helpWriter{
-		indent:             "",
-		width:              guessWidth(ctx.App.Stdout),
-		lines:              &lines,
-		HelpPrinterOptions: options,
+		indent:      "",
+		width:       guessWidth(ctx.Stdout),
+		lines:       &lines,
+		HelpOptions: options,
 	}
 	return w
 }
@@ -137,7 +143,7 @@ func (h *helpWriter) Print(text string) {
 }
 
 func (h *helpWriter) Indent() *helpWriter {
-	return &helpWriter{indent: h.indent + "  ", lines: h.lines, width: h.width - 2, HelpPrinterOptions: h.HelpPrinterOptions}
+	return &helpWriter{indent: h.indent + "  ", lines: h.lines, width: h.width - 2, HelpOptions: h.HelpOptions}
 }
 
 func (h *helpWriter) String() string {