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; ?>
```


*I'm losing the plot, somehow the extra two commits got added to this one. Arrgh. Anyway, commit https://github.com/molovo/anchor-cms/commit/ab2764fb450ed9bd8f03d4f61ba98a067ba3116b also adds a fix for issue #453. When creating the default post the content was wrapped in single quotes in the insert code, which meant that the newlines were taken as literal characters not parsed. Made it double quotes, ét voila.*