add flow type declarations

[n1ru4l]

No description provided.

[resir014]

I'm not familiar to Flowtype, but could these typings be extracted to a separate type, like my suggestion on the TypeScript PR?

Something like:

interface OnlineStatusOpts {
    onOnline?: () => void,
    onOffline?: () => void,
    onStatusChange?: (online: boolean) => void,
}

export default function useOnlineStatus(opts: OnlineStatusOpts): boolean;