Wikipedia: show HTTPS links without &oldid (Closes #232)

I'm not sure if this is the best long term solution for #232, but Wikipedia currently hard-codes the `http://` and `&oldid` parts into its `Retrieved from` link. I don't see any other way of fixing this issue without editing the link ourselves.

Bearing in mind though that websites' layouts do change at times, and even the slightest changes can completely break the plugin (#186, #226 as examples).

edit: This also really fixes #59.