Add Command.Aliases and deprecate Command.ShortName

`Aliases` will be more flexible while still allowing "ShortName"
behaviour via `Aliases`.

app.go

@@ -5,6 +5,7 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
+	"strings"
 	"text/tabwriter"
 	"text/template"
 	"time"
@@ -91,8 +92,12 @@ func (a *App) Run(arguments []string) (err error) {
 		}()
 
 		HelpPrinter = func(templ string, data interface{}) {
+			funcMap := template.FuncMap{
+				"join": strings.Join,
+			}
+
 			w := tabwriter.NewWriter(a.Writer, 0, 8, 1, '\t', 0)
-			t := template.Must(template.New("help").Parse(templ))
+			t := template.Must(template.New("help").Funcs(funcMap).Parse(templ))
 			err := t.Execute(w, data)
 			if err != nil {
 				panic(err)