New [subs:view] tokens and new 'Subscriptions' view mode

[subs:view] gives you rendered nodes and comments in one piece, that can be customized using the 'Subscriptions' view mode. This doesn't work too well for text-only mails, but I think it should be useful for creating HTML mails.

I've pushed this too the -dev version — let us know how it goes if you decide to give it a try.