Stringify BigInts

[ntr-808]

I ran into issues using https://github.com/trentm/node-bunyan which makes use of this library.

This PR simply converts BigInts to strings, which I think is reasonable because

> BigInt('11111111111111111111111111111111111');
11111111111111111111111111111111111n

BigNumber.js similarly uses strings as an intermediate representation.

[TwitchBronBron]

Any chance this could get merged and released soon?

[TwitchBronBron]

For those who are interested, I worked around this issue in rokucommunity/logger#3 by using the replacer 2nd argument.

const safeJsonStringify = require('safe-json-stringify');
const theObjectToStringify = {
    someBigInt: BigInt('123')
};
const jsonString = safeJsonStringify(theObjectToStringify, (key, value) => {
    //if it's a BigInt, return the string value instead
    return typeof value === 'bigint' ? value.toString() : value;
});