Fixed linking of DOIs.  Closes #163.
Fixed linking of DOIs.  Closes #163.

Now only a part of a prefix that begins a url (http...) will be
part of the linked text for a DOI.  (This part will also be used
for the URL, in place of the default http://doi.org/, if present.)

Other parts of the prefix and suffix will remain outside the
linked text.

jkr/pandoc-citeproc,jgm/pandoc-citeproc,adunning/pandoc-citeproc