Potential UnhandledPromiseRejection?

In your [readme example](https://github.com/kevva/gulp-decompress/blob/3e881dc0f39f4d9f67a926789f663a9998f52f0f/readme.md#usage) I believe there is potentially an UnhandledPromiseRejection.

The Transform stream has no error handler so I suspect the [synchronous call to the flush callback](https://github.com/kevva/gulp-decompress/blob/1af310982ac152f1cfccf74dde47732c8508a1d9/index.js#L46) will cause it to re-throw inside the promise scope.

Start with the usage example in readme.md and inspect index.js at line 46, where the synchronous flush callback is referenced. Reproduce the reported failure path and verify whether the Transform stream emits an unhandled rejection; the issue is complete when that error path is handled and the example no longer produces the warning.

Written by the indexing model from the issue text.