Merge pull request #1426 from urfave/default-command-doc

Approve v2 addition of App.DefaultCommand

@jalavosus FYI I'm merging this now with the intent to tag and push `v2.11.0`, and I'm happy to see further documentation improvements released later.

testdata/godoc-v2.x.txt

@@ -269,6 +269,9 @@ type App struct {
 	Version string
 	// Description of the program
 	Description string
+	// DefaultCommand is the (optional) name of a command
+	// to run if no command names are passed as CLI arguments.
+	DefaultCommand string
 	// List of commands to execute
 	Commands []*Command
 	// List of flags to parse