Add feature finish event

[robertkofoed]

Hey, I discovered that Winston wont emit 'finish'-event. And this package neither!
This is a problem for me in testing with Mocha, and in production with AWS Lambda's.
Now I can discover when its safe to terminate the process.

My Fix:

Example:

const logger = winston.createLogger({
    transports: [
        new (winston.transports.Console)({
            timestamp: true,
            colorize: true,
        })
    ]
})

const transport = new CloudWatchTransport(config)
logger.add(transport)

transport.on('finish', () => { console.log('finished!') })

How you might ask?

By:

relay.js line 63:
if (i === batch.length - 1) this.emit('finish')

index.js line: 16:
this._relay.on('finish', () => this.emit('finish'))

Which in turn allows me to use detect when the Lambda is safe to freeze