Fire callback to inform winston about log delivery in time

[markablov]

Hey,

process.on('uncaughtException', err => {
  logger.error('uncaughtException', err, {stack: err.stack});
  logger.on('finish', () => process.exit(1));
  logger.end();
});

In this code in case of uncaught exception we could lose logs, because winston-elasticsearch transport calls callback inside log() instantly, but it supposed to do it after log delivery (or an error, but it more complex ;) )

[coveralls]

Coverage increased (+1.7%) to 81.25% when pulling de6e51f on markablov:master into 51fa69c on vanthome:master.

[vanthome]

Very nice, wanted to do this also but could not find that the bulk writer has callback support immediately.