Make _LxmlFrameParser more extensible
Allow creating a `_LiberalLxmlFrameParser` (see conversation on: https://github.com/pydata/pandas/issues/4469#issuecomment-22142824)

For now I will factor out:

```
parser = HTMLParser(recover=False)
```

so that subclass can change the behavior. 

Would there be an issue with adding `_LiberalLxmlFrameParser` to the Pandas codebase?
