Consider supporting custom ssl certificates configuration for elasticsearch client

It would be really great to support the `transport_options` hash parameter for the Elasticsearch::Client initialization, to specify a custom SSL cert for `https://` urls.

See this section of the [Elasticsearch README](https://github.com/elastic/elasticsearch-ruby/blob/master/elasticsearch-transport/README.md#authentication) for example usage.

Not sure what your preferred method of [passing this configuration data](https://github.com/junegunn/redis-stat/blob/8eca2537d8897c59319660698728add4affc8012/lib/redis-stat/elasticsearch.rb#L27) into the `redis-stat` server is, @junegunn.

It feels like adding another CLI flag for this option would be clunky, and you don't have any precedent for a config file. What are your thoughts here?

I'd be happy to help with a PR, or could fork if you think this change isn't generally useful.

Thanks for such a great tool!

Read the Elasticsearch README authentication section and inspect lib/redis-stat/elasticsearch.rb, which is linked as an example of passing configuration. Determine how redis-stat should expose transport_options without relying on an unspecified config-file mechanism, then verify that a custom SSL certificate can reach the Elasticsearch::Client for HTTPS URLs.

Written by the indexing model from the issue text.