Allow to return promise of token from login function

[jcubic]

Expected behavior

• Feature that will allow to return promise from login or use async..await + return token.

Actual behavior

Right now if you use async you need to call the callback.

Steps to reproduce

function delay(value, ms) {
    return new Promise((resolve) => {
        setTimeout(() => resolve(value), ms);
    });
}
$('body').terminal($.noop, {
    login: async function(user, pass, callback) {
        var x = await delay({token: '4abxd9'}, 1000);
        return x.token;
    }
});

[jcubic]

Terminal should also pause when value from login is a promise.