Add new recipe for making streams from memory contents

Submitted a recipe on how to make streams with contents stored in variables.

I have a the example repo here : https://github.com/AntouanK/gulp-stream-test
Only problem I have with this approach is that task `write-versions` exits ok, but at the time it exits, the files are not written. ( add a console.log in [line 77](https://github.com/AntouanK/gulp-stream-test/blob/master/gulpfile.js#L77) where I resolve the promise and you'll see that it's printed after the task ends )
```bash
541 stream-test (master)$ rm -rf output/ && gulp
[09:57:20] Using gulpfile ~/CODE/stream-test/gulpfile.js
[09:57:20] Starting 'default'...
[09:57:20] Starting 'load-lib-files'...
[09:57:20] Starting 'load-versions'...
[09:57:20] Finished 'load-versions' after 7 ms
[09:57:20] Finished 'load-lib-files' after 16 ms
[09:57:20] Starting 'write-versions'...
[09:57:20] Finished 'write-versions' after 3.98 ms
[09:57:20] Finished 'default' after 21 ms
file written
file written
```
What's wrong with promises?
( I don't see any other easy way to know when to exit after starting many async streams... )