Keep track of bookmarks' content history

[bonswouar]

As discussed in #1128, now that text-screenshots are implemented it could be a great feature to have some revision history on bookmark contents. Afaik it shouldn't be hard to implement the basics. Although to make it really sexy it'd be nice to have some waybackmachine-like UX, and maybe to be able to configure few options (the first that come to mind: minimum time between saves, max save age / number of saves, and a list of customs tags to ignore for always-changing content like forums showing online users for example)