Only officially support node 8 and higher from Commander v4

[shadowspawn]

We have been planning for a long time to use more modern javascript features, but changes will break older versions of node and we do not have a documented policy.

Node 8 is the oldest version in LTS. I suggest we only officially support node 8 and higher from Commander v4.

However, I suggest we actually target node 6 when considering language features and not deliberately break node 6 until there is good reason. That should cover most people using a current version of Commander while node 6 usage drops off. We no longer run tests on node 6 though so it won't be officially supported.

[shadowspawn]

I think the README addition is useful to be clear about what we are testing, but based on #1073 I think in practice we will still need to bump major version if we deliberately make code changes to (say) require node 6 or 8. Without being noisy about the required version using engines we will still break people who might reasonably expect compatibility when updating within a major version.

• https://docs.npmjs.com/files/package.json#engines

[shadowspawn]

I noticed today that that I used class for defining new CommanderError so we have indeed started requiring newer version of node, and not just a future consideration! Class was fully supported from node 6.

• https://node.green/#ES2015-functions-class
• https://docs.npmjs.com/files/package.json#engines

I'll do a PR adding engines field set to require node 6 to be explicit that Commander v4 onwards requires node 6 or higher.

[shadowspawn]

v4.0.0 has been released.