get_output don't handle promises

[jcubic]

Issue summary

If you echo promise and call get_output you get [object Promise]

Expected behavior

I should get the value the promise is resolved to

Actual behavior

It return string [object Promise]

Steps to reproduce

function render(text, delay) {
    return new Promise(resolve => {
        setTimeout(() => resolve(text), delay);
    });
}

term.echo(() => render('lorem', 10));
term.echo('foo');
term.echo(() => render('ipsum', 10));
term.echo('bar');
term.echo(() => render('dolor', 10));
term.echo('baz');
setTimeout(() => { term.echo(term.get_output()); }, 500);

Found while writing unit tests for async echo, was not able to use get_output() in 1415082

[jcubic]

The code now works like this, if any of the lines resolve to a promise then get_output() will return a promise of a string.

[jcubic]

Ther is still an issue, get_output() should not render the output, but get what is already printed on the terminal.