fromPromise allow options like { deep: true }

[eduardofcgo]

Hello,

Currently, the fromPromise calls extendObservable with the { deep: false } option by default.

MobX's observable.map and observable.set use { deep: true }. Similarly, I believe most of the time we expect the value of the promise to be observable as well.

Maybe it makes sense to allow the user to pass their own options?

Manually converting the resolved value to an observable afterwards can be cumbersome.