Fixed error when calling 'couchapp' without args

I was getting an error when running couchapp on my windows vm, so I added a check that process.argv[2] is not null:

```
C:\>couchapp

C:\Users\finsterdexter\AppData\Roaming\npm\node_modules\couchapp\bin.js:64
  return _isUsingDirectoryConfig = (process.argv[2].trim() === "-dc");
                                                    ^
TypeError: Cannot call method 'trim' of undefined
    at isUsingDirectoryConfig (C:\Users\George\AppData\Roaming\npm\node_modules\couchapp\bin.js:64:53)
    at Object.<anonymous> (C:\Users\George\AppData\Roaming\npm\node_modules\couchapp\bin.js:81:7)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
    at startup (node.js:119:16)
    at node.js:902:3
```