Add new "metadata" property to schema

The "metadata" property is intended for the storage of additional
non-searchable document properties. This allows additional information
to be stored and displayed in search results without having to make
changes to the schema.

Note that it is non-searchable as index is set to "no" and it's of
type "object".
Merge pull request #199 from alphagov/add-details-property-to-schema

Add new "metadata" property to schema