javascript newlines in strings need to be preserved in include filters

when including javascript files (unlike markdown) replacing newlines will cause javascript syntax errors. given 

strings.js

```javascript
var x = "here is \n some \n newlines";
```

and test.jade

```jade
html
   body
      includes strings.js
```

it will produce this file that contains javascript syntax errors

```html
<html>
   <body><script>var x = "here is
 some
 newslines";
</script>
   </body>
</html>
```


this pull request contains a fix (only for javascript files, i don't know if other filters need this or not) as well as a unit test to ensure this behavior is preserved.