Introduce Javascript controls

[mistercrunch]

This allows power-users to perform intricate transformations on data and
objects using javascript code.

The operations allowed are "sanboxed" or limited using node's vm
runInNewContext
https://nodejs.org/api/vm.html#vm_vm_runinnewcontext_code_sandbox_options

For now I'm only enabling in the line chart visualization, but the plan
would be to go towards offering more power to people who can write some
JS moving forward.

[fabianmenges]

I have 2 concerns:

• We are exposing the internal data model to the users, thus making it difficult to ever change. Now they can implement complex transformations on the data, which we won't be able to automatically migrate.
• What does the development story for these JS snippets look like? Is there a way for us to provide a debug console or something?

[mistercrunch]

@fabianmenges thanks for the feedback. I agree that we should be extremely careful in what is exposed here, whatever goes in and out has to be treated like a public API, so no backwards incompatible changes should be allowed without release notes (if ever).

We do control exactly what is accessible in the sandbox. I pass the console object in the context by default so users can console.log all they want. For instance I also pass underscore and d3 in there since there's a lot of utilities users may want, though that means that if we ever want to go d3.v4 we would have to be careful in that regard somehow as we could break user reports.

After thinking this through I'm removing d3 from the context and keeping underscore.

[mistercrunch]

So this PR adds the new control but the control is not used anywhere. I'd like to merge this independently from the use cases which will come in future PRs in the geospatial space.

Anytime this control will be used, we should consider the fact that it locks whatever it hooks (input and output) from backwards incompatible changes.

[mistercrunch]

@fabianmenges mind taking another look? Here the control is not actually being used by any visualization (planning to use it on DECK.GL viz eventually), and the console concern is taken care of.

[fabianmenges]

LGTM