Implement link translation.

Implement link translation.

This patch implements the possibility to translate links.
It can be useful if use both planner and Org-mode, if you sometimes
copy text from one side to the other and you do not want to worry
about link syntax.  This patch allows links created with planner to
function correctly in Org-mode.  I would probably be simple to
implement the reverse conversion in planner as well.

This patch is a first shot at this functionality and very likely can
be improved.