isIndexable: Function to check if a URL is indexable
Description
This function checks if a given URL is indexable. Therfore it checks different rules:
Status-Code: Does the URl gives a correct Status-Code (200)
Canonical-Link: Is the canonical Link Element implementet, so the URL can be indexed?
Meta-Robots: Is the Meta-Robots blocking the ULR?
robots.txt: Is the robots.txt blocking the URL from crawling?