Improve mutate function

[sergiodxa]

• If no data call trigger against the key
• If data is a function call it passing current cached value
• Return data or throw error after function
• SWR returns bound mutate function with the key already defined

Resolves #155
Resolves #253

[jameslouiz]

This is exactly what I'm looking for. Great work!

In the mean time, for a bounded Mutate with ability to provide a function with access to the old data you can do this:

const useSWRWithBoundMutate = (key, fetcher, options) => {
  const { data, error } = useSWR(key, fetcher, options);
  const boundMutate = fn => mutate(key, fn(data));
  return { data, error, boundMutate };
};

[omerman]

This is exactly what I'm looking for. Great work!

In the mean time, for a bounded Mutate with ability to provide a function with access to the old data you can do this:

const useSWRWithBoundMutate = (key, fetcher, options) => {
  const { data, error } = useSWR(key, fetcher, options);
  const boundMutate = fn => mutate(key, fn(data));
  return { data, error, boundMutate };
};

Also what I'm looking for :)!
And it also gives you freedom of accessing the mutate outside react's scope( useSWR hook), because in many cases you would want to update just a part of a given response, and it is impossible to perform it outside react's scope with the current api.
#253

[shuding]

The implementation looks amazing!

And I really like the boundMutate idea. I think we can deprecate trigger (and maybe revalidate as well) in the future.

BTW, can you also add this new usage to the README?

[shuding]

Also as I commented in #262, I think we should remove the shouldRevalidate trick in https://github.com/zeit/swr/blob/edcfd265d3ad22901b203ecdb25a1fdc92254bf2/src/use-swr.ts#L112-L116 to avoid misunderstanding.

[sergiodxa]

@quietshu I updated the README and removed the shouldRevalidate, always default to true now. Also updated the Optimistic UI example.

[shuding]

Thank you @sergiodxa!

Close #262, #136.