Add plugin and process for putting content into search

The jekyll_rummageable plugin will parse the HTML generated during the
compilation process and create a JSON version of it suitable for pushing
into rummager.

Changes to compile.sh copy that JSON into designprinciples ready to be
part of its deploy process.

This has been tested end-to-end in a dev VM but should be reviewed on
preview. It may well be that we also want to amend the plugin to use the
raw markdown documents for generating the JSON rather than HTML, but this
approach was the simplest to implement for now.
Merge branch 'master' into add-search
Break up multiple blocks on single line

As @fatbusinessman pointed out some of this logic was hard to read.
Move responsibility for explaining output format to Page
Add missing newline to Gemfile
Correct comments to remove reference to plugin that inspired this one
Add apostrophe

Is this really that important? Must ask my therapist...
Merge pull request #43 from alphagov/add-search

Add search