As suggested by @osantana in #257, adding a --https config option.

Mostly for semantic reasons, I suggested keeping Marmite.https as Option<bool> instead of bool: this avoids a sort of incoherent state where url == "https://my.site" while https == false.

However I haven't implemented checks for other possible inconsistency (user can use --url https://my.site and --https false and in this case the https is ignored). This bit relies on the added docs (the help message for --https explicitly says it is used only if protocol is missing in the URL).

The changes in content.rs and site.rs are unrelated but suggested by cargo clippy — happy to revert, but I kept them since they are part of the contributing guide.