Fix: brace-style stroustrup should report on cuddled elseif (fixes #1583)

The `brace-style` rule with the `"stroustrup"` option on was failing
to report cuddled `elseif`s because of a check to make sure the
`alternate` type was `BlockStatement` before checking the position of
the brace tokens. For `elseif`s, the `alternate` type is `IfStatement`.

```
if (foo) {
  baz();
} else if (bar) {
  baz();
}
else {
  qux();
}
```

The above code will now report correctly.

Fixes #1583
Merge pull request #1593 from iancmyers/fix-1583

brace-style stroustrup should report on cuddled elseif