Feature request: async processing of each state upgrade

[guopengliang]

First of all, thank you for maintaining this library.

It would be more powerful if we could do async updates. In real cases, it is often complicated to upgrade/migrate database. Say we need to inject data loaded from another asyncStorage.

const manifest = {
    1: (state) => new Promise((resolve) => {
        // manipulate state object ...
        resolve(state);
    }),
    2: (state) => new Promise((resolve) => {
        // manipulate state object ...
        resolve(state);
    })
};

[rt2zz]

This seems like a sensible api, also async await would make the conversion to support Promises or plain return values trivial. Will gladly accept PR 👍