Add the `plugins` command to Jarvis

This add a new command to jarvis that allow you to get the status of the build for the default plugins that we ship with Logstash and it will periodically yield in the #logstash channel to tell us to look after them.

You can trigger the query manually like this:

@jarvis plugins

Jarvis Oops, We have currently *14* plugins jobs failing :sadbazpanda:
Failures for branch: *master*
logstash-codec-edn, logstash-codec-edn_lines, logstash-codec-es_bulk, logstash-codec-graphite, logstash-codec-json_lines, logstash-codec-multiline, logstash-codec-plain, logstash-filter-kv, logstash-filter-mutate, logstash-filter-split, logstash-filter-xml, logstash-output-elasticsearch, logstash-output-null, logstash-output-statsd

It will fetch the following file https://github.com/elastic/logstash/blob/master/rakelib/plugins-metadata.json to get the list of plugins.

It can check more than one branch for some plugins, you will have to make a PR to this repository and add your plugin and the additional branch to check to the plugins_config.yml file. like this:

logstash-output-elasticsearch:
  - "6.x"

This was motivated by https://github.com/logstash-plugins/logstash-input-beats/pull/203 and needed for the migration to jruby 9k

elastic / jarvis

Add the `plugins` command to Jarvis #76