Script field formatters

[rashidkpc]

After some discussion about how to implement some of the more advanced field formatters we'd like to implement the ability to use custom scripts to do formatting. The problem becomes, how do we do it safely?

1. All the user to type scripts into the web interface. Pros: Nice, simple, quick. Cons: Excessively unsafe. If there's a way to do this safely I've never seen it.
2. Require scripts be stored server side. Probably the right way to go. Pros: If you don't have access to the server, you can't add scripts. That's also the Con.

In theory this will already be possible due to #2731. The entirety of this ticket may involve adding a server API endpoint to list the available script formatters and make them available via a drop down in the UI

We need to spec out what the function should take, and if it should be angular-y. There are some issue with bringing in too much functionality here. For example if we allow it to inject html, or get ahold of the element containing the value we're going to have a hard time implementing server side chart rendering without something like PhantomJS.

[cjcenizal]

@rashidkpc Can you give some examples of the scripts you had in mind and the use cases for them?

[rashidkpc]

I think this is already done, someone should just document it. @timroes did a writeup of it for Kibana 4: https://www.timroes.de/2016/02/17/writing-kibana-4-plugins-field-formatters/