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

build.go

@@ -24,7 +24,7 @@ func build(k *Kong, ast interface{}) (app *Application, err error) {
 	if len(node.Positional) > 0 && len(node.Children) > 0 {
 		return nil, fmt.Errorf("can't mix positional arguments and branching arguments on %T", ast)
 	}
-	app.Node = *node
+	app.Node = node
 	app.Node.Flags = append(extraFlags, app.Node.Flags...)
 	return app, nil
 }
@@ -63,13 +63,13 @@ func buildNode(k *Kong, v reflect.Value, typ NodeType, seenFlags map[string]bool
 		ft := field.field
 		fv := field.value
 
-		name := ft.Tag.Get("name")
+		tag := parseTag(fv, ft)
+
+		name := tag.Name
 		if name == "" {
 			name = strings.ToLower(dashedString(ft.Name))
 		}
 
-		tag := parseTag(fv, ft)
-
 		// Nested structs are either commands or args.
 		if ft.Type.Kind() == reflect.Struct && (tag.Cmd || tag.Arg) {
 			typ := CommandNode
@@ -105,6 +105,7 @@ func buildChild(k *Kong, node *Node, typ NodeType, v reflect.Value, ft reflect.S
 	child := buildNode(k, fv, typ, seenFlags)
 	child.Parent = node
 	child.Help = tag.Help
+	child.Hidden = tag.Hidden
 
 	// A branching argument. This is a bit hairy, as we let buildNode() do the parsing, then check that
 	// a positional argument is provided to the child, and move it to the branching argument field.