Support passing time as BigInt

[IronGeek]

Hi, There!

This PR add support for passing the time argument as BigInt.

AS IS, when trying to pass a BigInt to pretty-time, for example:

const start = process.hrtime.bigint();
prettyTime(process.hrtime.bigint() - start);

Would result in:

TypeError: Cannot mix BigInt and other types, use explicit conversions
    at module.exports (pretty-time/index.js:32:19)

This PR resolves the above issue by first converting the BigInt value to the legacy process.hrtime() format ([number,number]) and then continue with the rest of formatting logic AS IS.

closes #2