React Suspense support

[kosich]

Enable Suspense for SSR

Something like

const App = () => {
  // $Suspense is a reactive element
  <$Suspense fallback={ <span>loading...</span> }>
    { stream$ }
  </$Suspense>
}

--

TODO: Consider approach suggested by @joshribakoff in https://github.com/kosich/react-rxjs-elements/issues/4#issuecomment-683872341 with BehaviorSubject