Scripts checking

WIP

Javascripts are another important source we could check. They behave technically mostly like images, except they can have content. Should I implement more?
I'd rather this be rewritten as:

``` ruby
if script.missing_src?
  self.add_issue "script is empty and has no src attribute"
else
  next if script.ignore?
  if script.remote?
    add_to_external_urls script.src
  else
    self.add_issue("internal script #{script.src} does not exist") unless script.exists?
```