Writable instead of through stream for partials

Hello,

As through stream used for partials never gets read, it blocks on 16 (highWaterMark from rvagg/through2 library) items. This means that plugin can't support more than 16 partials at the moment. I can assume there is the same limit for helpers, but I didn't check.

This pull request introduces simple writable stream based on isaacs/readable-stream, that is used instead.

The other option I considered is just adding 'data' event listener on original through stream. That works as well, but it felt a bit too dirty.

If you'd like anything changed, just shout.

Cheers,
Dali