Initialize _version, _versionOptionName in constructor

The _version and _versionOptionName properties are initialized to
undefined in the constructor. The reasoning is
- to make them visible when the Command instance is logged into the
console before .version() is called, and not only appear after suddenly
- to not break anything if the proxy use for consistent option value
handling in the legacy setOptionValueWithSource mode suggested in tj#1919
(and previously in tj#1921) is adopted

lib/command.js

@@ -77,6 +77,9 @@ class Command extends EventEmitter {
     this._helpCommandnameAndArgs = 'help [command]';
     this._helpCommandDescription = 'display help for command';
     this._helpConfiguration = {};
+
+    this._version = undefined;
+    this._versionOptionName = undefined;
   }
 
   /**