flag.domains with mailchecker at build-time

[FGRibreau]

https://dl.red.flag.domains/red.flag.domains_fr.txt

[YasharF]

Just a thought, should this be during the build-time, install time, startup time, or a combination (i.e. have fallbacks)? By moving the data download closer to the startup/runtime the consumers can have more up-to-date lists without having to constantly download/upgrade the package.

[FGRibreau]

Good point, maybe it should be at install time by default and optionally activated at run-time I think.

The download URL should point to https://github.com/FGRibreau/mailchecker/blob/master/list.txt and we could leverage github actions to automatically update that lists and merge it with https://dl.red.flag.domains/red.flag.domains_fr.txt ?

If that's something you are interested in doing, I'm open for a PR :)