Similar to FileArchive.add but accepts html strings for substitution

def add(self, obj=None, filename=None, data=None, info={}, html=None):
        "Similar to FileArchive.add but accepts html strings for substitution"
        initial_last_key = list(self._files.keys())[-1] if len(self) else None
        if self._auto:
            exporters = self.exporters[:]
            # Can only associate html for one exporter at a time
            for exporter in exporters:
                self.exporters = [exporter]
                super(NotebookArchive, self).add(obj, filename, data,
                                                 info=dict(info,
                                                           notebook=self.notebook_name))
                # Only add substitution if file successfully added to archive.
                new_last_key = list(self._files.keys())[-1] if len(self) else None
                if new_last_key != initial_last_key:
                    self._replacements[new_last_key] = html

            # Restore the full list of exporters
            self.exporters = exporters