Change CHANGELOG format for next release, and add pending changes

[shadowspawn]

We could adopt "Keep a Changelog" format for our CHANGELOG.

This is what it looks like rendered:
https://github.com/shadowspawn/commander.js/blob/feature/keepachangelog/CHANGELOG.md

I only use "Unreleased" on the develop branch. When we are preparing the release before we merge to master, we put in the release version number and date.

I put the date in brackets as it is the most common style used on GitHub.

I have added the links to make the Pull Requests clickable when viewing the CHANGELOG directly. It is a little tedious adding them, but does make the references more useful.

All the link definitions are at the bottom of the file so they are out of the way when reading the plain text.

The version numbers in the headings (and "Unreleased") are clickable links which do a compare in GitHub to see the changes from last release. Again, adding the URLs is a bit manual but it is an easy copy-and-edit after the first one.

Example of a section with a version number:

## [3.0.0] (2019-08-08)

[3.0.0]: https://github.com/tj/commander.js/compare/v2.20.0...v3.0.0

Rendered markdown:

3.0.0 (2019-08-08)

[abetomo]

That's a nice job.