Plugins should define their internal version separately from the Kibana version

[epixa]

While 5.0+ plugins must specify the exact version of Kibana that they work with, they also may have their own versioning scheme. We should continue to require that plugins specify a Kibana version, but they should also be able to specify their own internal version in a consistent way, and we should render that internal version when listing plugins via the CLI or status page.

[pickypg]

Currently the status page lists the version as reported by the plugin. I think it seems reasonable for us to require a kibana object that specifies specific for-Kibana attributes, such as:

{
  "version" : "1.2.3",
  "kibana" : {
    "version" : "5.0.0-alpha4"
  }
}

• Plugin's version; may or may not differ from the Kibana version
• kibana.version; must be the same for the running version of Kibana

Then, kibana.version is checked for strict equality against the current Kibana version. version is then only really used to inform users what they're running so that they can refer back to the plugin author for help given the version info.

Elasticsearch behaves in the same way as above.