classes with negative values are not generated when used with prefix

[barnabasmolnar]

As per the title.

When using a prefix, eg tw-, classes with negative values (such as tw--mx-2) aren't generated. Works fine without the prefix.

Is this a known limitation or just a minor oversight?

[barnabasmolnar]

Apologies, I've just noticed that there's an open issue about this already. Closing in favour of #42.