Pass an array as an argument

To pass an array of streams to `merge-stream`, we have to write like below, now.

```javascript
var gulp = require('gulp');
var merge = require('merge-stream');

gulp.task('css', function(){
  var tasks = [/* some tasks */];
  return merge.apply(null, tasks);
});
```

But it seems not intuitive for beginners. I'd like to change it little bit like below. How about?

```javascript
return merge(tasks);
```
@grncdr Pls review this row. Your code returned `source`, but in terms of chaining it seems better to return `this`?