add new filter to modify assets path

Hi!

Thanks for amazing script

I write a small improvement - in my osclass site i mixed http/https protocols (https only on login, register, and other pages).
But all assets js, css files have hardcoded http protocol in url.
Writing in core is badly but allow to modify by filters is ok :smile: 
```
str_replace(array('http://', 'https://'), '//', $url)
```

Example output:
```
<link href="//oc-content/themes/xxx/css/main.css" rel="stylesheet" type="text/css" />
```