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
```

However,

```jade
// This is supported by wiredep:
// bower:js
// endbower

// This is not:
//- bower:js
//- endbower
```

wiredep allows us to use a custom regexp in order to support the latter, but it is not very convenient imho.

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).