Nunjucs recipe: Compile to .tmp instead of app

Keeping compiled stuff out of the source directory. :+1: 
Maybe it would be better if you add the rest of the task to avoid confusion:

```diff
gulp.task('html', ['styles', 'templates'], function () {
  var assets = $.useref.assets({searchPath: '{.tmp,app}'});

- return gulp.src('app/*.html')
+ return gulp.src('app/*.html', '.tmp/*.html')
    .pipe(assets)
    .pipe($.if('*.js', $.uglify()))
    .pipe($.if('*.css', $.csso()))
    .pipe(assets.restore())
    .pipe($.useref())
    .pipe(gulp.dest('dist'));
});
```