time: allows signs for year/tz in format string

The following program prints `1994-01-01 00:00:00 -0059 -0059`:

```
package main

import (
    "fmt"
    "time"
)

func main() {
    ret, err := time.Parse("06Z070000", "-6+-1+1-1")
    if err != nil {
        panic(err)
    }
    fmt.Printf("%v\n", ret)
}
```

Most of the RFCs does not allow that.
