Adds a `Destination` field for the `TimestampFlag` type that allows you to specify a pointer to
a `Timestamp` rather than having to grab the `Timestamp` from the `cli.Context` using the flag
name.
expect(t,err,fmt.Errorf("invalid value \"2006-01-02T15:04:05Z\" for flag -time: parsing time \"2006-01-02T15:04:05Z\" as \"Jan 2, 2006 at 3:04pm (MST)\": cannot parse \"2006-01-02T15:04:05Z\" as \"Jan\""))