* Make any frontmatter <scheme>:// URI's clickable
http/https links are opened in a new tab/window (via target="_blank"),
other schemes are opened in the current tab, as the browser will prompt
the user to open another application (or do this automatically).
Per https://www.rfc-editor.org/rfc/rfc3986#section-3.1, the scheme must
follow the pattern:
scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
* Make `mailto:...` strings clickable in frontmatter
Zef Hemel