unbuffered comments support for Jade

Jade supports [two types of comments](http://jade-lang.com/reference/comments/):

```jade
// this is a buffered comment, it will appear as <!-- ... --> in your HTML
//- this is an unbuffered comment, it will not appear in your HTML
```

I added the ability to use both types of comments by default, so we can use unbuffered comments and be consistent with [gulp-inject](/klei/gulp-inject).