Support passing in a fetch object? #34

Been using node-fetch-cache and big fan, but a few pieces of feedback. I'll file them as separate PRs from what we've run into. Thanks for consideration!

It would be nice to be able to pass in a fetch object to use vs the hard-coded fetch.

that would allow using something like https://www.npmjs.com/package/fetch-retry underneath and caching the final results.