Add entry value as a parameter to defaultValue function

[JonathanSun]

Why am I submitting this PR

This replaces the skipDefaultValues and useKeysAsDefaultValues with a more powerful option to use value itself as a function param.

Example:

defaultValue: (locale, namespace, key, value) => value ? value : key

Does it fix an existing ticket?

Yes #677

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• tests are included and pass: yarn test (see details here)
• documentation is changed or added

[karellm]

Thanks a lot, the PR looks great! I've added two small comments.

[karellm]

Published as 7.0.0