Add support for $stealth modifier

For example:
```
@@||data.simpleupload.net/download/$stealth
@@||ingka.com^$stealth,domain=ikea.com
@@||s*.bpwco.com^$xmlhttprequest,stealth
```

![image](https://user-images.githubusercontent.com/29142494/110925539-9dd4e000-8323-11eb-9154-e1b3634ad7b7.png)
