Add in attachments on the government index

Using Tika to preprocess the attachments you can store an array of the
attachment text to be included in the search index.

We are not using the attachments-mapper plugin because this stores the
base64 encoded version on the file (twice) and IG has 40GB of files

Extend the attachment model to include extra fields to index
Merge pull request #134 from alphagov/index-attachments

Add in attachments on the government index