add purgecache.yml to purge jsDelivr cache

[piquark6046]

Please check the uBO internal discussion.

[gorhill]

https://github.com/jsdelivr/jsdelivr#purge-cache:

To avoid abuse, access to purge is given after an email request

As stated, I prefer to not use this purge feature unless a extremely convincing argument has been made that this should be done here. At the moment, diff-updates are becoming more and more widespread, will see in two months where things stand with regard with filter list update issues.