refresh() instance method on PromiseState

I'm new to the library but i've been using the manual refresh pattern from the readme a lot because each of my screens have a refresh button.
```
connect(props => {
const url = `/users/${props.userId}`

return {
userFetch: url,
refreshUser: () => ({
userFetch: {
url,
force: true,
refreshing: true
}
})
}
})(Profile)
```
Reading through the documentation for [PromiseState](https://github.com/heroku/react-refetch/blob/master/docs/api.md#promisestate) i noticed that there is a refresh() static method but not an instance method. I think adding it as an instance method would be hugely helpful to a lot of people. It would reduce the above to:
```
connect(props => ({
userFetch: `/users/${props.userId}`,
})(Profile)
```
and then call the refresh using `props.userFetch.refresh()`

Am i missing something? It doesn't look like anyone has mentioned it before, do they just write the extra code?