Add pagination support for sections and taxonomies

A new configuration property `paginate` is added with default value `0`. This setting will generate the site with no pagination, as it is today.

Setting `paginate` to a positive value will split the list pages for sections and taxonomies into chunks of size of the `paginate` property.

`Data.Paginator` is provided as a template variable to help building a pager menu.

The pages are built on the form:

```
[SECTION/TAXONOMY]/index.html
[SECTION/TAXONOMY]/page/1/index.html => redirect to  [SECTION/TAXONOMY]/index.html
[SECTION/TAXONOMY]/page/2/index.html
....
```

Fixes #96