Add options to specify indices and ignore errors

Specify indices to search on (default to logstash-*) for #95 and option to ignore errors from @greem in #57.

Helps with ```[No mapping found for [@timestamp] in order to sort on]``` when using kibana with dashboards stored in elasticsearch
This config is a bit of a misnomer. Currently, logstash fails and generates a very verbose error message, and this config (if invoked) will instead simply inject a new field: "query_failed" with the contents being the query string. The logs get pretty cluttered currently.