Adds a given index to the Pipfile.

def add_index_to_pipfile(self, index, verify_ssl=True):
        """Adds a given index to the Pipfile."""
        # Read and append Pipfile.
        p = self.parsed_pipfile
        try:
            self.get_source(url=index)
        except SourceNotFound:
            source = {"url": index, "verify_ssl": verify_ssl}
        else:
            return
        source["name"] = self.src_name_from_url(index)
        # Add the package to the group.
        if "source" not in p:
            p["source"] = [source]
        else:
            p["source"].append(source)
        # Write Pipfile.
        self.write_toml(p)