Request: have grunt automatically enter async mode if task returns a promise

Every grunt task I've ever seen looks, essentially, like this:
```
grunt.registerTask('build', function () {
const done = this.async();

callSomeModule()
.then(() => done())
.catch(error => {
grunt.fail.warn(error);
done(false);
});
});
```

It would be nice if _by default_, grunt recognized a Promise return value and did the boilerplate for you (spitting a stack trace to `warn` if the promise is rejected, continuing if the promise resolved).

I'd be open to to putting up a tentative PR, if this sounds like something that could be added.

Example desired syntax:
```
grunt.registerTask('build', function () {
return callSomeModule();
});
```

Start at the grunt.registerTask entry point and trace how task completion currently handles this.async() and returned values. Define and test the behavior for resolved and rejected Promise returns, including continuation and warning stack traces; done means the example syntax works without explicit async boilerplate.

Written by the indexing model from the issue text.