ENH: read-html fixes

slight change to exception

fix assertions start on additional classes to support html5lib and html.parser

mark slow parsers' tests as slow

use strainer's attribute for table parsing in bs4 subclasses

add better testing data

do not search for thead and tfoot for now

add ground truth data

update read-html

fill out subclass methods

ENH: refactor read-html to allow html5lib backend

add skip

make skip slightly more general

TST: add test to convert to and from df.to_html

CLN: sanitize names

ENH: read-html fixes

DOC: minor doc fix