Router exact to pass down to Route

[c0ldra1n]

Little extra, but this will enable writing:

<Router exact>
  <Route path="/">
    <Index />
  </Route>
  <Route path="/hello">
    <Hello />
  </Route>
  <Route path="*">
    <NotFound />
  </Route>
</Router>

instead of:

<Router>
  <Route exact path="/">
    <Index />
  </Route>
  <Route exact path="/hello">
    <Hello />
  </Route>
  <Route exact path="*">
    <NotFound />
  </Route>
</Router>

Edit: the build failed even without the changes I've made-- not sure why. This is my first pull request, so please let me know if there's anything I've done wrong. Thank you! 😃