Add url entry to bibtex

Addresses #499.

Author: arunkannawadi

scholarly/publication_parser.py

@@ -402,6 +402,11 @@ def bibtex(self, publication: Publication) -> str:
             publication = self.fill(publication)
         a = BibDatabase()
         converted_dict = publication['bib']
+        try:
+            url = publication['eprint_url']
+        except KeyError:
+            url = publication.get('pub_url', '')
+        converted_dict['url'] = url
         converted_dict = remap_bib(converted_dict, _BIB_REVERSE_MAPPING)
         str_dict = {key: str(value) for key, value in converted_dict.items()}
         # convert every key of the dictionary to string to be Bibtex compatible