Adds quick link to 'like' posts

I hope this is the right way to add a migration.

I've been asked how to create something like this a few times and figured it's worth including since it's so simple.

Adds a route to enable 'likes' on posts. Direct user to `like/slug`, and the value in the likes column is incremented by one, then the session is updated to reflect the fact that the post has been liked, so that the link can be disabled.

Implement like so:

```php
<?php if (Session::get(article_slug()) == 'liked'): ?>
    <a href="#">Thanks!</a>
<?php else: ?>
    <a href="<?php echo Uri::to('like/' . article_slug()); ?>">Love it!</a>
<?php endif; ?>
```