Add a simple elastic search check

Adding a really simple elasticsearch health check. It parses the output from the `_cluster/health` which I think it semi-useful to get a quick idea about the cluster status.

I'm also thinking of parsing the node stats that come from `_cluster/nodes/stats?all=true` however I'm not sure if it's best to include that within the same tool, or make it a separate tool - perhaps with a toggle for `all=true`.

What do you think?