Add UsageOnMissing option to Kong

Adds an UsageOnMissing option which configures Kong to omit errors and
display usage when a command is invalid due to a missing argument. This
can be useful if a user wishes for the top-level command to print usage
when no args are supplied, or if they would prefer child commands which
have their own subcommands to print usage specific to that command.

Signed-off-by: hasheddan <georgedanielmangum@gmail.com>

kong.go

@@ -47,13 +47,14 @@ type Kong struct {
 	resolvers []Resolver
 	registry  *Registry
 
-	noDefaultHelp bool
-	usageOnError  bool
-	help          HelpPrinter
-	helpFormatter HelpValueFormatter
-	helpOptions   HelpOptions
-	helpFlag      *Flag
-	vars          Vars
+	noDefaultHelp  bool
+	usageOnError   bool
+	usageOnMissing bool
+	help           HelpPrinter
+	helpFormatter  HelpValueFormatter
+	helpOptions    HelpOptions
+	helpFlag       *Flag
+	vars           Vars
 
 	// Set temporarily by Options. These are applied after build().
 	postBuildOptions []Option