dim: merge in minor correctness fix
===================================
`HTMLParser.handle_starttag` and `HTMLParser.handle_startendtag` now expect
`List[Tuple[str, Optional[str]]`, acknowledging the fact that attribute values
may be None. Our implementation does not expect None, so we convert None to
the empty string.
https://github.com/zmwangx/dim/commit/8d71282d1d036c57385cd2e4310616220076fe3c