when generating, don't overwrite files

Say you have a view generator with a template dependency:

    {
      "files": [
        { "from": "view.js.hbs", "to": "app/views/{{name}}.js" }
      ],
      "dependencies": [
        { "name": "template", "params": "{{name}}" }
      ]
    }

If a template with that name already exists, it gets overwritten.