Writable instead of through stream for partials

As through stream used for partials never gets read,
it blocks on 16 (highWaterMark) items. This means that
plugin can't support more than 16 partials without
change.
add failing test to verify https://github.com/TakenPilot/gulp-static-handlebars/pull/2
Merge branch 'tes-more-partials' into with-seventeen-partials
Merge pull request #3 from TakenPilot/with-seventeen-partials

With seventeen partials