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
Worth documenting that the value here is whatever the ES 'multiple indices' feature supports: http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/multi-index.html