Use post's directory path when matching for the post_url tag

Adds support for matching file names of posts in subdirectories. Works with the following:

```
{% post_url 2013-04-25-post-name %}
{% post_url /2013-04-25-post-name %}
{% post_url blog/2013-04-25-post-name %}
{% post_url /blog/2013-04-25-post-name %}
```