Avoid pasting from the blackhole register

[sosmo]

Yankstack's mappings (line 170) rely on Vim's v:register variable to define the currently active register. However, there's one oddity about the v:register: after yanking/deleting something its value doesn't update until the cursor is moved or some other action is performed (try doing "_x:echo v:register, apparently it's intended). That means that if you delete something into the blackhole register for instance, and then immediately use Yankstack to paste without specifying a register, the paste_with_key function receives the previously used _ in the parameter a:register instead of " which is what the user would expect. In other words, Yankstack will try to paste from the blackhole register instead of the default register with p if the previous command used the blackhole reg.

The commit simply transforms the blackhole register into default register when pasting. This fixes pasting with plain p so you can do "_xP for instance. Pastes using an explicit register are unaffected, apart from the blackhole register which I don't think anybody has any reason to paste from.

I haven't tested the commit with the suite since I'm unfamiliar with Ruby and kind of preoccupied. It's been working for me for a while though and it seems like a harmless change.