feat: Expose a "verbose" mode for generated programs.

[parkerziegler]

Currently, the programs we generate obscure some of the details of how constants were produced. For example, when we generate breaks in choropleth scales it's clear that we use a given method (e.g., quantiles, Jenks, etc.), but it isn't apparent from the program what code led to those values being produced. For users who want a deeper understanding of how inlined constants are produced, including those calls in a "verbose" mode could be a good option. This would still give users the option of getting an "optimized" program in which all constants are inlined.

• Add a "verbose" flag to our codegen process that indicates we should retain code used to generate constants inline.
• Add support for codegen'ing choropleth color scales using d3.