Add quick fix to change unused variable to _

``` go
package main

import "fmt"

func demo() (int, int) {
    return 1, 2
}

func main() {
    a, b := demo()
    fmt.Printf("a:\n", a)
}
```

Currently `b` is reported as unused but there could be a quick fix to change it to `_`:

``` go
    a, _ := demo()
```
