update generator to use new docgen method from 1.1.7

This basically brings in the changes to docgen from logstash 1.1.7, which seem to provide better and more accurate parsing of plugins (multi-line config options, constants used as valid values for a config option, etc)

It's basically these logstash commits (and the associated fixes around them):
* logstash/logstash@8cbb10bb4a1dea034fc852135b401ab26db09760
* logstash/logstash@1d4d4c137a3c0fdc85d63fc524e6de7accb27644