Generate default configuration documentation programmatically

This is PR just to generate some discussion about @ZDroid's comment in #2102. What I've done here is create a liquid tag that outputs the default configuration for Jekyll. But I'm not too happy about this solution since this exposes a tag to all Jekyll users that they wouldn't really use. So again, this PR is just a conversation starter and not really a complete work. Several alternatives I've been thinking about is:

- Extract this tag into its own gem and add it in the gemspec just like `jekyll-coffeescript` or `jekyll-sass-converter`. So at least the code is somewhere else. But it's the same issue because the tag is still publicly available.
- Make this a plugin and generate docs offline before pushing to `gh-pages` branch (since you can't run plugins when using Github to parse Jekyll).
- Expose `Configuration::DEFAULTS` to liquid variables. Maybe `site.default_config`?
- Instead of listing the defaults, link to the Github source code.
- Just keep doing what is done now which is synchronizing the doc with Jekyll source manually. 

Thoughts?