Message is not sent if shutdown immediately

Nobody has claimed this yet.

# Overview

Since this module doesn't have shutdown function, request to slack is not reached if `log4js.shutdown()` is called and process is terminated right after message event.

For example:
https://github.com/expressjs/generator/blob/master/templates/js/www.ejs#L68
When I replace this logging with log4js + slack appender, it will not notify to slack.
Even if I put `process.exit(1)` within shutdown callback like
```
const log4js = require('log4js');
const logger = log4js.getLogger();

logger.error(bind + ' requires elevated privileges');
log4js.shutdown(() => {
process.exit(1);
});
```
it still won't wait process.exit() since slack appender doesn't support it.

As seen in log4js-node, it checks function existence.
https://github.com/log4js-node/log4js-node/blob/master/lib/log4js.js#L117

# Appendix

If following proposal diff looks good, I'll make Pull Req.
https://github.com/log4js-node/slack/compare/master...honeroku:feature-support-shutdown

It doesn't guarantee that all request is done, but I suppose 1sec is enough (or even too much) for slack api.
And I don't want over-decorate this tiny module, so I made minimum support.

Start with the Slack appender and the linked feature-support-shutdown comparison, then review how log4js.shutdown() checks for shutdown support. Verify the immediate-termination example and the proposed callback behavior; done means the Slack request can finish or be given the proposed wait before process exit.

Written by the indexing model from the issue text.