Issue title: Ability to render custom tags
 Issue body: So basically, I am using mistune to store my blog posts. And sometimes I have some images or iframes or other tags that I only use in that article. So I want to be able to render customly, using some syntax like : `~img-href"link"alt"image"~` and it would be parsed as `<img href="link" alt="image"`. It is not just for that tag but any custom tag we want to make, like `~anytag~` render as `<anytag>`. 
 Comments: 
 Comment 0: You can always use HTML in Markdown.
{'source': 'GithubIssues', 'repo_name_with_owner': 'lepture/mistune', 'issue_id': 281028254, 'repo_stars': 2130.0}