Feature: Router param types

[mojombo]

Want to constrain a route param to look like a number and then auto-convert it into an actual JS number before passing it on? BOOM:

<Route path="/order/{num:Int}" page={OrderPage} name="order" />

// Given path `/order/3` the params would be `{ num: 3 }`
// Notice that 3 is an integer, not a string!

The above is actually shorthand for doing it the long way:

<Route
  path="/order/{num}"
  page={OrderPage}
  name="order"
  constraints={{ num: /\d+/ }}
  transforms={{ num: parseInt }}
/>

That's cool and all, but what if you have your own constraints and transforms that you want to apply in multiple routes? Oh, Redwood can do that:

const paramTypes = {
  slug: {
    constraint: /\w+-\w+/,
    transform: (x) => x.split('-'),
  },
}

const Routes = () => {
  return (
    <Router paramTypes={paramTypes}>
      <Route path="/post/{id:slug}" page={PostPage} name="post" />
      <Route notfound page={NotFoundPage} />
    </Router>
  )
}

// Given path `/post/super-duper` the params would be `{ id: ['super', 'duper'] }`

Core param types like Int start with an uppercase letter, and user param types start with a lowercase letter. This means we can add core types without ever conflicting with anyone's user types. Tada!

[peterp]

Awesome!

[mojombo]

This PR doesn't yet handle the per-route constraints and transforms but I'm going to save that for a later PR so we can get the tutorial functionality working right away.