Improve handling of absolute paths

Given an HTML file containing a css resource using an aboslute path:
`<link rel=stylesheet href=/css/site.css>`

Result:
```
$ htmlark mypage.html
**SS: Error reading '/css/site.css': No such file or directory
Unable to convert webpage: [Errno 2] No such file or directory: '/css/site.css'**
```

I think htmlark should be able to handle this better.