Add cors option

Usually stubcell is requested by other port, so I added CORS option.

```
var stubcell = new StubCell();
stubcell.loadEntry(__dirname + "/cors.yaml", {basepath: "", debug: true, cors: true});

//or 

stubcell.loadEntry(__dirname + "/cors.yaml", {basepath: "", debug: true, cors: {
  "Access-Control-Allow-Origin": "example.com",
  "Access-Control-Allow-Method": "GET",
  "Access-Control-Allow-Headers": {"someField": true}
}});

```


Now set "false" by default, but I think it is better to set "true" by default.


I think this.cors option would be better to be `true`.