Make channels configurables

With this modifications you can configure (but not required) which content will be used for generating the rss channel.

For example we can use document content with layout for the description or use the mtime for the item date.

By default, the generated content is still the same as before this PR.

A full configuration will be
```coffee
      default:
        collection: 'html'
        url: '/rss.xml'
        maxItems: 10
        channel:
          title: (templateData) -> templateData.site.title
          description: (templateData) -> templateData.site.description
          pubDate: (templateData) -> templateData.site.date
          author: (templateData) -> templateData.site.author
        item:
          title: (document) -> document.title
          description: (document) -> document.contentRenderedWithoutLayouts
          date: (document) -> document.date
          author: (document) -> document.author
```