adds permalink formatting options

Adds ability to specify permalink format in YAML frontmatter. Supports a few different uses:

```permalink: date``` a predefined style :year/:month/:day/:title
```permalink: :year/:title``` a custom formatter
```empty (no permalink attribute)``` behaves same as before

### Predefined template

input file: ```/articles/hello-world/index.md```

    date: 2013-08-03
    permalink: date

output file: ```articles/2013/08/03/hello-world/index.html```

### Custom template

input file: ```/posts/hello-world/index.html```

    date: 2013-08-03
    permalink: :year/:title

output file: ```posts/2013/hello-world/index.html```

### With filename option

input file: ```/articles/hello-world/index.md```

    date: 2013-08-03
    filename: foo.html
    permalink: date

output file: ```articles/2013/08/03/hello-world/foo.html```

### Works anywhere

input file: ```/hello-world.md

    date: 2013-08-03
    permalink: date

output file: ```2013/08/03/hello-world.html```