feat: Add whitelist extend option
This PR introduces a new, optional config option WHITELIST_EXTEND. If it is set to true, the whitelist provided by IFramely will be extended with the domain entries from the local whitelist instead of replacing it, while continuing to regularly update the remote Iframely whitelist and watch for changes in the local whitelist folder.
By default WHITELIST_EXTEND is set to false and doesn't change the current behavior, so this is no breaking change.
We haven't had time to accurately impact-assess and test this PR against the whitelist lifecycle and hierarchy in v1.4.0.
However, here's a question and suggestion: why don't you simply merge whitelists on the server? You could replace our iframely.com/qa/whitelist.json service with your own. It could pull from our central list and redefine entires per your PR, but all on your side. Without any risks and extensive testing on our end.
Thanks again for the PR. I am going to close it without merging. If there's still a need, please merge JSONs somewhere externally and then pass Iframely the merged file.