Add safe argument

[AndreyFrolov44]

Hi!

I suggest adding the safe argument, in which url_fetcher will be used, in which it will be impossible to download local files

[liZe]

Hi!

Thanks for this pull request.

Unfortunately, I don’t think that adding a safe option is a good idea, because it may give the impression that using it solves all the security concerns. Avoiding access to local files is only one small part of all the possible security problems that may happen with WeasyPrint, and there’s no silver bullet to avoid them all.