Remove CategorizedHelp from App and allow subcommands to have categories

Just place all subcommands in categories, the default category will be
"" which will properly format the output (and group commands that have
no category).

Also allow subcommands to have categories.

Lastly, augment the test to check the output.

command.go

@@ -3,6 +3,7 @@ package cli
 import (
 	"fmt"
 	"io/ioutil"
+	"sort"
 	"strings"
 )
 
@@ -231,6 +232,13 @@ func (c Command) startApp(ctx *Context) error {
 	app.Email = ctx.App.Email
 	app.Writer = ctx.App.Writer
 
+	app.categories = CommandCategories{}
+	for _, command := range c.Subcommands {
+		app.categories = app.categories.AddCommand(command.Category, command)
+	}
+
+	sort.Sort(app.categories)
+
 	// bash completion
 	app.EnableBashCompletion = ctx.App.EnableBashCompletion
 	if c.BashComplete != nil {