Implement Nested Menu as Promise

[josetaira]

This commit allows you to pass a promise as your Nested Menu.

[Templarian]

Now I didn't actually have time yet to read your commit, just merging and am review it as I test it out. 😄 (my other open source project is taking all my time, but I don't want to ignore this, just cutting corners).

[josetaira]

@Templarian no worries. I tried to make as small a change as possible but still implementing the required features. Shouldn't be too difficult to test it out.

Have you considered adding other people with write access to this repo to help you out?

[Templarian]

@josetaira Good idea! You're a contributor now (probably should have done this sooner, but these github emails get buried by all the activity of my main project). Open an issue when you think it's ready for a version bump and I'll increment and release. We just need to make sure we don't break the current user's stuff and that the docs are updated.