Update node support from v8, v10 to current releases (v12, v14, v16)

[jwhitlock]

The README says:

This is tested on and should run on any of Node.js {8, 10}.

The current node.js releases (July 2021) are:

• v12 - Maintenance LTS (end-of-life 2022-04-30)
• v14 - Active LTS (maintenance starts 2021-10-19)
• v16 - Current (active LTS starts 2021-10-26)

PR #95, updating mocha from v8 to v9, is failing tests because it expects v12 at a minimum.