Add option for document expiration

[oxr463]

If the published date is older than a predefined interval, then mkdocs build should fail, and a warning message issues for the offending document.

For example, here is a snippet from an article

---
date: 2001-07-19
---

# Article Title

And here is the option in my mkdocs.yml file:

extra:
  expiration_period: 2Y

Then during the build phase:

mkdocs build
Error: documentation has not been update since July, 2001.

[zhaoterryy]

I would like to keep this plugin as simple as possible. The feature you're asking for should be a separate plugin. Thanks for the suggestion!