Fix single element lists in config

Currently if you try and use a list containing a single element like ```['foo']``` in the configuration file(s), Logstash interprets that as access to a field literally called ```'foo'``` in an event, (including the quotes). By updating the ```selector_element``` pattern in the grammar to not match either ```'``` or ```"``` in field names Logstash now correctly interprets this as a list.

I updated the ```in``` & ```not in``` operator tests to cover this.

Fixes [LOGSTASH-2201](https://logstash.jira.com/browse/LOGSTASH-2201).