Suggestion for changelog / process

I also kept forgetting to add changelog entries (also using the same changelog format). I added this to package.json / scripts:

```
"preversion": "grep \"\\[$npm_package_version\\]\" CHANGELOG.md > /dev/null || ( echo 'You need to add an entry in CHANGELOG.md for this version.' && false )"
```

If you don't have entry for the version you're trying to add with npm, then the command will fail. It's worked for me reliably.

*Context*: the current version is 2.0.5, but this isn't tagged or referenced in Github, and the changelog file is several versions out of date. For anything that uses newish browser APIs, it's often useful to do manual checks on things that have changed, even minor/bug fix version changes.

I can open a PR with this change if you agree that this is a useful addition.