Switched parser to htmlparser2

This is the minimal amount of code necessary for the switch. There is still some potential for optimizations; I did some of that work [last year](https://github.com/fb55/jsdom/commit/ed26682cf67a5ea013d21216f657418653324531), but as there were a couple of tests I couldn't get working, I dropped it. I recently rewrote the parser from scratch using a FSM, and now everything seems to work nicely.