I've refactored `org-export-as-html', factored code to build links
into `org-html-make-link'.

This does two things that I needed:

 * It allows custom link types to build anchors.
   * How: Call org-html-make-link.  Many parameters, see the function
     docstring.  It returns a string containing an HTML link.
 * It adds the capability to convert links when exporting.
   * How: Around the export call, bind org-html-cvt-link-fn to a
     function that takes 1 parameter (filename) and returns a url as a
     string.

I think it also makes the code cleaner.

There are more things that could be done - it's only used by some of
the cond branches, the others are unchanged.  But "publish early and
often", so here it is.

I will append the changes as a diff, since I can't push to the org
repository ("fatal: The remote end hung up unexpectedly")

	Tom Breton (Tehom)