title: Logstash - Setting "config.support_escapes" hasn't been registered
body: <p>I did follow the description of the official elastic.co documentation when I was configuring the ELK stack. As per the <a href="https://www.elastic.co/guide/en/logstash/master/configuration-file-structure.html#_escape_sequences" rel="nofollow noreferrer">https://www.elastic.co/guide/en/logstash/master/configuration-file-structure.html#_escape_sequences</a> page shows how to set Logstash to enable escape characters, I just uncommented the line config.support_escapes: true in my logstash.yml.
  When I start my ELK stack in my Docker container,I get the following error:  </p>
  
  <pre>
  An unexpected error occurred! {:error=># ArgumentError: Setting "config.support_escapes" hasn't been registered>,
   :backtrace=>["/opt/logstash/logstash-core/lib/logstash/settings.rb:32:in `get_setting'",
  "/opt/logstash/logstash-core/lib/logstash/settings.rb:64:in `set_value'",
  "/opt/logstash/logstash-core/lib/logstash/settings.rb:83:in `merge'", 
  "org/jruby/RubyHash.java:1342:in `each'",
  "/opt/logstash/logstash-core/lib/logstash/settings.rb:83:in `merge'", 
  "/opt/logstash/logstash-core/lib/logstash/settings.rb:135:in `validate_all'",
  "/opt/logstash/logstash-core/lib/logstash/runner.rb:244:in `execute'", 
  "/opt/logstash/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:67:in `run'",
   "/opt/logstash/logstash-core/lib/logstash/runner.rb:209:in `run'", 
  "/opt/logstash/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:132:in `run'",
   "/opt/logstash/lib/bootstrap/environment.rb:71:in `(root)'"]}
  </pre>
  
  <p>What did I miss here?</p>