Adds API.route_param for quick declaration of route parameters.

I've found myself often implementing this pattern:

```ruby
namespace :users do
  namespace ':id', requirements: {id: /[0-9]+/} do
    get { }
    put { }
  end
end
```

And thought that it might be useful to implement a little bit of syntactical sugar for it:

```ruby
namespace :users do
  route_param :id, requirements: /[0-9]+/ do
    get { }
    put { }
  end
end
```

Thoughts?