[added] <Routes handlerProps = {…} >

Anything you pass into handlerProps will be passed as props into all
your handlers.

For instance:

  <Routes
      handlerProps =  {
                        {
                          "favoriteFruit": "banana",
                          "favoriteDrink": "smoothie"
                        }
                      }
  >
    <Route handler = { Trunk } >
      <Route handler = { Branch } >
        <Route handler = { Leaf } />
      </Route>
    </Route>
  </Routes>

if Leaf was the active route; Trunk, Branch, and Leaf would all be
called with favoriteFruit and favoriteDrink as props, like so:

  <Trunk
    favoriteFruit = "banana"
    favoriteDrink = "smoothie"
  >
    <Branch
      favoriteFruit = "banana"
      favoriteDrink = "smoothie"
    >
      <Leaf
        favoriteFruit = "banana"
        favoriteDrink = "smoothie"
      />
    </Branch>
  </Trunk>