ol.el: add description format parameter to org-link-parameters

[Hugo Heagren]

Suggested patch to add a new link parameter controlling how the
default description is generated. As explained in the commit, this
behaviour is often similar between links of the same type, but differs
between those types, so a parameter seems like a good place to specify
it.

I've tried to make `org-insert-link' behave sensibly with regard to
all the possible options -- hope it's alright.

Blue skies, Hugo