Add option to unmarshal JSON with UseNumber

[stetra]

This PR adds SetUseNumber which enables the use of UseNumber to prevent precision loss when unmarshalling integers.

Example usage:

func main() {
	o := orderedmap.New()
	o.SetUseNumber(true)
	json.Unmarshal([]byte(`{"x":9007199254740993}`), &o)
	x, _ := o.Get("x")
	fmt.Println(x)

	// Output: 9007199254740993
	// If UseNumber is not called, x will be 9007199254740992.000000
}