Enable html_link_suffix

I want to control the extension for internal links.
For example, if you hide the extension using the MultiViews feature of Apache.
This is a patch for that.

Fixes summary
- Change to `app.config.html_link_suffix` from hard code '.html'(Sphinx has `html_link_suffix`)
- Add `html_link_suffix` to `MetaData` class
- Append `html_link_suffix` to `metadata.link`