Add pipe condition

Let's say I have a `less` task and I want to add `sourcemaps` depending on `environment`. 

## This is original `less` task with no environment support:
```
gulp.task('less', function() {
  gulp.src('./dev/src/**/*.less')
    .pipe(sourcemaps.init())
    .pipe(less())
    .pipe(sourcemaps.write())
    .pipe(gulp.dest('./.tmp'));
});
```

## This is common solution:
```
gulp.task('less', function() {
  var res = gulp.src('./dev/src/**/*.less');
  if (env.DEBUG === 'yes') {
    res = res.pipe(sourcemaps.init());
  }
  res = res.pipe(less());
  if (env.DEBUG === 'yes') {
    res = res.pipe(sourcemaps.write());
  }
  res = res.pipe(gulp.dest('./.tmp'));
});
```

## This is my propose solution, more elegant (I think): 
```
gulp.task('less', function() {
  gulp.src('./dev/src/**/*.less')
    .pipe(env.DEBUG === 'yes' ? sourcemaps.init() : null)
    .pipe(less())
    .pipe(env.DEBUG === 'yes' ? sourcemaps.write() : null)
    .pipe(gulp.dest('./.tmp'));
});
```

## Overal idea: 
If you have the `pipe` like this:
```
gulp.task('less', function() {
  gulp.src('./dev/src/**/*.less')
    .pipe(null)
    .pipe(less())
    .pipe(null)
    .pipe(gulp.dest('./.tmp'));
});
```
Then it will execute like this:
```
gulp.task('less', function() {
  gulp.src('./dev/src/**/*.less')
    .pipe(less())
    .pipe(gulp.dest('./.tmp'));
});
```