refreshInterval as function

[correttojs]

currently the refreshInterval option can only be a number. In some cases this number should be dynamic, dependent on the updated fetched data.
For example, the fetcher returns a list of items and a value corresponding to the next expected update.

[nilskaspersson]

I would appreciate this functionality as well, and in particular would I like to reuse the exponential back-off behavior used in onErrorRetry. My use-case is that I occasionally have eventually consistent data, but no way of knowing when. Usually it's within seconds, but sometimes hours—but I only need it conditionally.

I'm happy to draft a PR, but I would like to see some discussion on it first, to see if it's even wanted in the first place.

[shuding]

This is now landed in 1.1.1 (release). 🎉