SEO improvement: Canonical url support for .md/.html rendering

[dkarvounaris]

Since files can be rendered with the .md as also .html extension (.html is the default when internal links are created from .md files) this can be essentially duplicate content for Google&Co, if someone creates links as example from external sources explicitly with .md instead of .html or even without any extension which also works.

A canonical URL gets away with duplicate content by telling search engines one URL to use for any other that renders the same output.

Support is planned in Version 2.0.0.