Large integers lose precision when parsed

[remorses]

When parsing command-line arguments with large integer values (> Number.MAX_SAFE_INTEGER), precision is lost because the value is converted to JavaScript Number.

mri(['--id', '9007199254740993'])
// { id: 9007199254740992 }  // wrong! lost precision

This affects downstream packages like cac which uses mri for argument parsing.

Proposed fix: Use BigInt for integers that exceed the safe integer range. See #28