setFlushInterval support for Javascript Mode

[Tofufu]

It looks like setFlushInterval() has been implemented in mixpanel-config.js: https://github.com/mixpanel/mixpanel-react-native/blob/master/javascript/mixpanel-config.js#L88, however, it doesn't work out-of-the-box yet with Javascript Mode as it's not defined in e.g. index.js and else where.

Request is to have this functionality supported (There is a thread in our #sdks for this)