Add quickfix to add default zero values to returns

There's a tool called goreturns, https://github.com/sqs/goreturns, which allows a code like this:

``` go
func F() (*MyType, int, error) {
    return errors.New("foo")
}
```

to become this:

``` go
func F() (*MyType, int, error) {
    return nil, 0, errors.New("foo")
}
```
