Add local healthcheck

This is way faster than the normal one, so we can get faster feedback
Health check support for multiple backend types

Changed the health check script to be a command line script rather than
a rake task as it's easier to accept and parse command line options.

I've used a gem called 'slop' for command line parsing:
  https://github.com/injekt/slop

Added the html and json backends to support the old ways of running
health check. This should mean we can delete the govuk-search-health
repo.
pin slop gem version
$LOAD_PATH is more readable than $:
Mention health check in readme