(maybe) Support a `--secure-only` flag, that warns about http (vs https) links

[hoijui]

Problem

Outdated content often has http://... links instead of the https://... versions,
which almost all sites support by now. It would be good if they were changed.

Is the same possible in other cases?
Looking at ftp vs sftp, for example, I think we could not do the same,
as one can not generally assume that there is sftp where there is ftp,
nor that it points to the same content.

Solution

With something like a --secure-only flag,
mlc issues a warning for each http (unencrypted) link.

Alternatives

It would be quite trivial to do this manually using grep -e '^http://' on a list of exported, gathered links.
Personally, I prefer this way of doing it.

[becheran]

@hoijui I am not against this. Though, I also don't think this there is a lot of need for this feature.

If websites do not support http anymore, they will (if done right) just redirect http to https. If the server still offers http without https, it is more a server config issue.