LaTeX export: Use special formatting by link type interface

LaTeX export: Use special formatting by link type interface

Patch by Christian Moe, who writes:

> It looks like support for formatting custom link types in LaTeX export
> is broken?
>
> I was trying to implement a custom link type with its own formatting
> function for HTML and LaTeX export, following the steps in
> org-bbdb.el.
>
> I've found that org-bbdb-export does not italicize bbdb links in
> LaTeX, nor does my own org-cite-export turn my custom =cite:= links
> into LaTeX =\cite{}= citations. Everything works fine in HTML export,
> but in LaTeX all custom link types get formatted as =\texttt{descr}=.
>
> I see that org-export-as-html and org-export-as-docbook look up
> org-link-protocols to get the function for formatting the link, but it
> seems that org-export-as-latex doesn't.
>
>