Add ability to split content in include files.

With this pull request, users can now include extra markdown files in the rendered result.

The list of files to be included should be in the collection names _includes_ at the top of the index.md file, in the order that they should be included. The content from the included files will be added after the content from the index.md file.

The included files should be created in the source/includes folder and be named with a leading underscore (e.g. source/includes/_errors.md).