PSR for Slugifier/Urlizer Interface

Hi,

Just to let you know, we are making a proposal for the creation of a standard slugifier/urlizer interface at fig-standards:

```
https://github.com/php-fig/fig-standards/pull/100
```

There is also a mailing list post:

```
https://groups.google.com/forum/?fromgroups=#!topic/php-fig/J-6s9Wlyk-A
```

It would be cool if this PSR goes through and we can apply it to this library :)

Cheers

Dan
