Updates for the collection options.

The collection options can now be configured to sort ascending and descending and, optionally, use the page data context properties for sorting. See #204 for more information.

```js
assemble: {
  options: {
    collections: [
      {
        title: 'tags',
        sortorder: 'desc'
      }
    ]
  }
}
```

The pages can also be sorted. I haven't tested this enough and I think it's still buggy, but I should have it soon.

```js
assemble: {
  options: {
    collections: [
      {
        title: 'pages',
        sortorder: 'desc'
      }
    ]
  }
}
```

I'll keep this PR open and continue to work on it.