Support 'script' key to shortcut to arbitrary executable

Allows 

```json
{
    "name": "Launcher",
    "icon": "${MENU_DIR}/launcher.ico",
    "script": "node-webkit/nw.exe",
    "scriptargument": "${PREFIX}/node-webkit/launcher"
  }
```

to create a shortcut which executes `$PREFIX\node-webkit\nw.exe $PREFIX\node-webkit\launcher`.