Download pictures of one hashtag.

        To download the last 30 pictures with hashtag #cat, do::

            loader = Instaloader()
            loader.download_hashtag('cat', max_count=30)

        :param hashtag: Hashtag to download, without leading '#'
        :param max_count: Maximum count of pictures to download
        :param post_filter: function(post), which returns True if given picture should be downloaded
        :param fast_update: If true, abort when first already-downloaded picture is encountered

def download_hashtag(self, hashtag: str,
                         max_count: Optional[int] = None,
                         post_filter: Optional[Callable[[Post], bool]] = None,
                         fast_update: bool = False) -> None:
        """Download pictures of one hashtag.

        To download the last 30 pictures with hashtag #cat, do::

            loader = Instaloader()
            loader.download_hashtag('cat', max_count=30)

        :param hashtag: Hashtag to download, without leading '#'
        :param max_count: Maximum count of pictures to download
        :param post_filter: function(post), which returns True if given picture should be downloaded
        :param fast_update: If true, abort when first already-downloaded picture is encountered
        """
        hashtag = hashtag.lower()
        self.context.log("Retrieving pictures with hashtag {}...".format(hashtag))
        count = 1
        for post in self.get_hashtag_posts(hashtag):
            if max_count is not None and count > max_count:
                break
            self.context.log('[{0:3d}] #{1} '.format(count, hashtag), end='', flush=True)
            if post_filter is not None and not post_filter(post):
                self.context.log('<skipped>')
                continue
            count += 1
            with self.context.error_catcher('Download hashtag #{}'.format(hashtag)):
                downloaded = self.download_post(post, target='#' + hashtag)
                if fast_update and not downloaded:
                    break