refactor JS code to separate modules

[timelyportfolio]

After converting to d3v4, I would like to modularize the JS code to resemble Let's Make a D3 Plugin. As an example of this, I have done d3horizonR. d3horizonR is not perfect, but demonstrates how the JS lives in its own separate directory ignored by the R pkg. The JS gets processed, built, and compiled and then moves to the inst/htmlwidgets folder for use by the R package. The advantage is we can stick to a "modern" typical JS build and test workflow.

See leaflet as another example.

Potential Modules

A quick round of code analysis reveals some clear module opportunities.

• HTMLWidgets module, but this should be separate from the main JavaScript build
• Vis layout module
• Breadcrumb module
• Legend module
• Hierarchy builder and helper module