Add dates to changelog entries

Automated with this script:

```bash
git tag -l | sed 's/^v//g' |
    xargs --replace bash -c 'sed -r -i "s/^({})$/\1 - $(git show --format=%ad --date=short --no-patch v{})/g" CHANGELOG.md'
sed -r -i 's/^(=+)$/\1=============/g' CHANGELOG.md
                     # - 2019-01-01
```