resolve DOI entries using https

[Bruce Ravel]

In format_doi in style/formatting/unsrt.py it would be nice to make the following edit so that DOI entries resolve to https URLs, as suggested by the DOI System.

    def format_doi(self, e):
        # based on urlbst format.doi
        return href [
            join [
                'https://doi.org/',
                field('doi')
                ],
            join [
                'doi:',
                field('doi')
                ]
            ]

[Andrey Golovizin]

Done, thanks.

[Hong]

As well as pubmed and arxiv....