Adding support for optional collections

Adding support for optional collections.

In the grunt file, collections can be specified in options:

```js
assemble: {
  options: {
    collections: ['archives', 'posts']
  }
}
```

In page yaml headers:

```yaml
---
archives:
  - 2013
  - May
  - 12
---
```

`tags` and `categories` are defaults and will always be included as collections.