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/

This allows to write [[wp:Org_mode][Org-mode]] instead of the
equivallent [[https://en.wikipedia.org/wiki/Org_mode][Org-mode]].