Added support for Request ID generators

[slavafomin]

When processing HTTP requests, it's a good practice to specify a unique request ID value for each log entry. This way, multiple log entries could be grouped together according to the specified request ID, which dramatically helps in debugging/analysing
requests processing.

This PR allows to specify custom request ID generation function for logger instances. Which opens a path for easy cls-rtracer integration. Please see the updated README for more details and examples.

This should be considered a non-breaking minor release.

---

• added support for Request ID generators
• introduced optional req_id property that can be passed to the logger constructor
• added tests
• updated README
• updated .gitignore file to exclude IDEA files
• arranged AUTHORS alphabetically
• fixed "engines" property of the package manifest
• added myself as an author

---

So I'm not sure regarding the maintenance status of this project, I've created and published a forked version of the library, so it could be used straight away if someone would want to.

[slavafomin]

Hello @trentm, could you look into this, please?

[LewisCowlesMotive]

Feels like it should be separate to a logger, and bound to .child

[jmaver-plume]

Any updates on this?

[jmaver-plume]

For anyone who wants to use something similar, you can take advantage of the get accessor property.

const logger = bunyan.createLogger({ ...options })
Object.defineProperty(log.fields, 'traceId', {
  enumable: true,
  get: function () {
    return 'some-id' // e.g., you can retrieve it from AsyncLocalStorage
  }
})