CONTINUATION_ALIGN_STYLE does not respect USE_TABS

I'm switching my project from space indentation to tabs using yapf `USE_TABS = True`.

Some of the existing code deals with overlong lines using backslashes

```
bar = {'baz': {'qux': {'quux': ''}}}
bar['baz']['qux']['quux'] \
= 'foo'
```

I want the second line to be indented with one tab, rather than the three spaces it currently is, so I set `CONTINUATION_ALIGN_STYLE = VALIGN-RIGHT`

Yet, yapf leaves this block of code as-is.