Org reader: add support for custom link types

Org allows users to define their own custom link types.  E.g., in a
document with a lot of links to Wikipedia articles, one can define a
custom wikipedia link-type via

    #+LINK: wp https://en.wikipedia.org/wiki/%s

This allows to write `[[wp:Org_mode][Org-mode]]` instead of the
equivallent `[[https://en.wikipedia.org/wiki/Org_mode][Org-mode]]`.

The current implementation allows only "%s" as a placeholder, while Org
also allows "%h" (the latter 'hexifies' non-URL characters).