Rake task to delete a slug from search

As part of changing a mainstream slug.

See:
https://github.com/alphagov/wiki/wiki/Changing-GOV.UK-URLs#making-the-change
Protect the rake task from nil or empty args
Make sure slugs begin with a leading slash
Merge pull request #230 from alphagov/feature/rake-task-to-delete-a-slug

Rake task to delete a slug from search