New html matchers: toContainText & notToContainText

Hi, I found that toHaveText html matcher doesn't really work as expected with html like this:
```
<div>
  Some text
</div>
```

Because it compares inner text exactly, including whitespaces. I added `toContainText` to deal with this problem. It's also useful for many other use cases. Hope you like it :)

Best,
Kris