added option to choose thread to run callbacks in. defaults to main

[spaul5]

No description provided.

[cbguder]

Thanks for the PR!

It feels like this should not be the concern of the promise library, but maybe I'm missing something. What's the use case for this? Can this not be handled in the callback like:

client.getValue().then { value in
    queue.async {
        doStuff(value)
    }
}

Also, even if there is a good use case, the queue being an argument to resolve feels backwards. Imagine your API client returning a future to a controller, and resolving the future later. I would argue the controller should decide where to handle the callback, not the API client. Fixing this would likely require storing a reference to the queue alongside the callbacks.