Add Command parent property to TypeScript (#1475)

Author: shadowspawn

typings/index.d.ts

@@ -156,8 +156,8 @@ declare namespace commander {
 
   interface Command {
     args: string[];
-
     commands: Command[];
+    parent: Command | null;
 
     /**
      * Set the program version to `str`.

typings/index.test-d.ts

@@ -21,6 +21,7 @@ expectType<commander.Command>(commander.createCommand());
 // Command properties
 expectType<string[]>(program.args);
 expectType<commander.Command[]>(program.commands);
+expectType<commander.Command | null>(program.parent);
 
 // version
 expectType<commander.Command>(program.version('1.2.3'));