html: improve parsing of lists

Make a <li> tag close the previous <li> element.
Make a </ul> tag close <li> elements.

Pass tests1.dat, test 33:
<!DOCTYPE html><li>hello<li>world<ul>how<li>do</ul>you</body><!--do-->

| <!DOCTYPE html>
| <html>
|   <head>
|   <body>
|     <li>
|       "hello"
|     <li>
|       "world"
|       <ul>
|         "how"
|         <li>
|           "do"
|       "you"
|   <!-- do -->

R=nigeltao
CC=golang-dev
https://golang.org/cl/5321051
2 files changed