Include the format in search documents

We should include the format of the document so that it can be used in the
search index for various things, such as weighting the format up and down.

More specifically/imminently, we want to display the format in search results
to give users context on what the search result is when HMRC Manuals and
sections appear in search results.
Merge pull request #78 from alphagov/include_format_in_search_documents

Include the format in search documents