[FEATURE]: Generate list ISO country codes (and country names) during topojson build process

[camdecoster]

Description

The list of available ISO country codes (and names) should be saved during the topojson build process such that the list can be included in the plotly.js documentation.

Why should this feature be added?

The default locationmode option for the scattergeo trace (and others) is "ISO-3" and the default topojson source is the maps shipped with plotly.js. As such, the list of ISO codes should be included in the documentation for users to reference.

Notes

• The country names should be included as well in the list. This will be for reference only.
• When locationmode is set to "country names", plotly.js uses a library to return the ISO code for a given country name. The names from the list in the point above aren't used during this matching process.