message: Add backend option to option checker
diff: @@ -5,7 +5,7 @@ const BaseServer = require('./base-server')
  const AVAILABLE_OPTIONS = [
  'subprotocol', 'supports', 'timeout', 'ping', 'root', 'store', 'server',
  - 'port', 'host', 'key', 'cert', 'env', 'bunyan', 'reporter'
  + 'port', 'host', 'key', 'cert', 'env', 'bunyan', 'reporter', 'backend'
  ]
  yargs
  