Allow flavor argument to read_html to be list/instance of _HtmlFrameParser
Currently the `flavor` argument to `read_html` must either be `None` or a string or a container of strings. This make is difficult for the user to write his own instance of `_HtmlFrameParser` and to use that when parsing. I suggest also allowing the user to pass in a subclass of `_HtmlFrameParser` in place of a string.

Implementation wise, this should be easy. `_parser_dispatch` would be modified to for this case. Likewise `_validate_parser_flavor` would need to allow this.
