Adding `on_activate` event on process, to notify when it get activated.
For instance, if a user opens the route_a which depends on route_b, and
after that he goes back to route_a, route_a won't render again, but a
series of events must to performed. This is where the `on_activate`
comes in. When a process is running or when a view is already rendered,
and another route asks for it, it'll pop out this event.
Dispatching the `on_activate` event just for the last active process in
the array instead of every active process.
Setting crawler flag into the window instead of the `@the` instance.
Making single results being iterated as well.
Releasing 0.2.13