Enable int64 option for msgpack-lite

[dbrgn]

64 bit integers are now wrapped with the int64-buffer library.

To try it yourself:

const data = new Uint8Array([
  131, 164, 116, 121, 112, 101, 166, 117, 112, 100, 97, 116, 101, 167, 115, 117, 98, 84, 121, 112, 101,
  174, 99, 111, 110, 110, 101, 99, 116, 105, 111, 110, 73, 110, 102, 111, 164, 100, 97, 116, 97, 130,
  162, 105, 100, 196, 32, 1, 2, 3, 4, 5, 6, 7, 8, 1, 2, 3, 4, 5, 6, 7, 8, 1, 2, 3, 4, 5, 6, 7, 8, 1,
  2, 3, 4, 5, 6, 7, 8, 166, 98, 105, 103, 105, 110, 116, 207, 127, 255, 255, 255, 255, 255, 255, 247
]);
const codec = msgpack.createCodec({int64: true});
decoded = msgpack.decode(data, {codec: codec});
console.log('Object', decoded.data.bigint);
console.log('Number', +decoded.data.bigint);
console.log('String', decoded.data.bigint.toString());

Fixes #91.