Add discover option to plugin command

It is not uncommon for us to release newer versions of plugins outside of the full Logstash release schedule. So it is quite possible that a newer version of a plugin (with bug fixes/enhancements) is available before the next full LS release is out. It will be cool if there is a (for example) `./plugin discover` command admins can periodically run, which will go and check for any later (and compatible) plugin versions that have been released (eg. for the plugins that are currently installed), and report back a list of available ones so that the admins can then determine when it is appropriate to run the plugin update command to update them. So this new option will just be a check-for-updates and listing command. Certainly, will require an online machine to be able to fetch the list of latest plugins available. Just a thought.