Hooks Support and Improvements

• We included hooks support: 🎉

import { useSplit } form 'react-splitio';

const [feature1, config] = useSplit('feature1');

if (feature1 === 'on') {
  return <Feature1 />;
}

• Changed client to be nullable.
  Now we don't have to fake a default client ({} as SplitIO.Client).
  This will prevent error if client is not defined and we try to call getTreatmentsWithConfig.
• Changed lastUpdate to just be 0 instead of null for default.
  Since it is a unix timestamp check 0 functions fine.
• Setting lastUpdate to "now" when the client is ready, because I'm pretty sure that's the time it has the treatments loaded from server.
• We're not re-creating the client if the config options have changed.
  By giving all the config options as a dependency list to useEffect.
• Converted SplitProvider to a function component.