Accept a list of ElasticSearch URLs

[boncheff]

The --url option should accept a list of URLs in case ElasticSearch is runing in a cluster

./elktail --url "s1:9200,s2:9200" -n "500" -t "timestamp" -f "%timestamp : %service : %message :" service:myservice.*

ERROR: Could not connect Elasticsearch client to http://s1:9200,s2:9200: no Elasticsearch node available.

[knes1]

Verified that elasticsearch client library olivere/elastic.v2 easily supports URL lists so this should not be problematic to add with minor changes to configuration options and options parsing. I'm starting work on this.

[boncheff]

Sounds good, thank you