Add 'in' support

if "somevalue" in [somefield] ...
- Add tests to cover the 'in' conditional operator. Anything that
responds to .include? should be valid. The following are roughly
synonymous:
  logstash config:  whatever in fancypants
  ruby: fancypants.include?(whatever)

  This operator is false if the object does not respond to the
  'include?' method.