@elseif is supported?

Hi, I found out the similar issue below.
https://github.com/jonathantneal/postcss-advanced-variables/issues/8

But it appears that @if and @else works fine, not @elseif.
Is there anyone to checkout this issue.

```
$theme-type: test1;

@if $theme-type == test1 {
background: #000000;
} @else if $theme-type == test2 {
background: #DDDDDD;
} @else {
background: #FAFAFA;
}
```

By the way, this plugin is so great.
Thanks for providing the great plugin!