enhancement: custom drilldown links

[mmguero]

It would be very useful to be able to add custom drilldowns to Kibana. There are a few examples of this via the elastic.co distribution and (not well-maintained) third-party plugins:

• elastic.co Kibana feature documentation (not open source)
• elastic.co github issue
• third-party plugin on github worked okay prior to new Kibana plugin architecture, hasn't been updated for a while

The feature would allow values to be used in creating URLs to other dashboards or external sources. For example, a dashboard developer could:

• generate a VirusTotal URL for a hash
• generate lookup URLs to iana.org for services, ports, protocols, mime-types, etc.
• generate links to issue ticketing systems, management tools, GitHub, etc.

This would allow a much greater degree of interoperability by allowing users to pivot to other tools from Kibana more easily.