feat(fetcher): retry middleware (#29)
A new middleware for retrying fetch requests when `response.ok` is
`false`. This middleware was designed to work on a per-endpoint basis.
This means that it should be added immediately after the endpoint
function.
```
import { createApi, requestMonitor, fetcher, fetchRetry } from
'saga-query';
const api = createApi();
api.use(requestMonitor());
api.use(api.routes());
api.use(fetcher());
const fetchUsers = api.get('/users', [
function* (ctx, next) {
// ...
yield next();
// ...
},
fetchRetry(),
])
```
It also supports a backoff function as a parameter. The function
signature is `(attempt: number) => number`, the result of which is how
long to delay the next fetch attempt. If a negative number is provided
then we stop retrying.
