Add async mutation support

[shuding]

With this change, the data parameter of mutate can be a promise, which returns the data or throws an error:

mutate(key, Promise<Data>)

It can simplify scenarios like

const newData = await update(...)   // some POST APIs return the updated data
mutate('/api/data', newData, false) // disable the revalidation here

to:

mutate('/api/data', update(...)) // POST + mutate w/o revalidation

Related: #93.

[cryptiklemur]

You can await a non-promise too. Not sure if there is a performance issue though

Suggested change

	if (_data && typeof _data.then === 'function') {
	const data = await _data;

[shuding]

It will defer the execution order (await non-promise values), similar to process.nextTick. But the major reason is that for promises (which should be mutation requests), we don't have to revalidate again after the request in most cases.

[pacocoursey]

Great feature idea! The actual code looks good to me.