Refactored the code to support almost any reference for any tag.

Useful to detect resources that could be local files or links. Also, it will throw errors or warning based on the tag type.
SCRIPT and LINK will throw errors.
A and IMG will throw warning.

For now let's just skip HTTP and HTTPS links and think that they're always working.
Match anything that can be placed from the tag to the src or href attribute.
Adapt the README to the new code.