--ignore-sass-mixins doesn't work with imported files

If a sass file imports another, `--ignore-sass-mixins` doesn't work within the imported file.

``` css
// file1.scss
@import "file2"
```

``` css
// file2.scss
@mixin foo {
  border: 1px;
  outline: 1px;
  padding: 1px;
}

h1 {
  @include foo;
}

h2 {
  @include foo;
}
```

`csscss --ignore-sass-mixins tmp/file1.scss` won't ignore mixins but `csscss --ignore-sass-mixins tmp/file2.scss` will.
