Retaining the unnamed register

In response to: https://answers.launchpad.net/ultisnips/+question/241600

Hey @SirVer, after thinking about that once again, it appears I was overcomplecting the problem.
It actually seems pretty straight-forward to implement at a low cost.

It can probably refactored a bit, I might lack understanding of the ultisnips codebase of course.
The logic needed looks a little twisted probably, but it works.

I am afraid I can't add tests right now - I can't get the test suite running, it just hangs and does nothing, whether trying it on my branch or master - need to investigate.