Feature: Add configuable option for managing allow or deny list of providers

[pa-eps]

This feature request is to add config options to the plugin to exclusively manage the following:

• a list of 3rd party providers that are allowed
• a list of 3rd party providers that are not allowed

For example: if the plugin is configured with certain provides to be "not allowed", then the plugin won't contact those service providers for embeddable objects.

Things to consider:

• What would be the behaviour when the plugin encounters this option?
  Exit with an error or just show warning? I think we should just write a warning and let remarkable process rest of the markdown contents.

• What would be the replacement content in the generated html page?
  Leave the original URL as-is or remove it entirely from the page?

• Do we need both "allowed" and "not allowed" list?
  Perhaps just one of them as an option is okay.