der: support negative integers

[jablko]

Before

const INTEGER = asn1.define('INTEGER', function() {
  this.int();
});
console.log(INTEGER.decode(INTEGER.encode(-1))); // Error: non-byte EncoderBuffer value at: (shallow)

After

console.log(INTEGER.decode(INTEGER.encode(-1))); // <BN: -1>
console.log(INTEGER.decode(INTEGER.encode(-0x81))); // <BN: -81>