Add asyncValue function

[dxinteractive]

I find that now dataparcels can handle saving but not loading, that it seems strangely asymetrical. It wouldn't be hard to write a function that loads initial value from a promise:

// loadData returns a promise

useParcelState({
    value: asyncValue(loadData())
});

Can also pass down valueStatus (like how asyncSubmit passes down submitStatus). Can even contain a updateValue() function to update as though updateValue is true

[dxinteractive]

Released in v1.0.0