Yank and paste between buffers

This hangs registers off of `atom.workspace.vimState` instead of `VimState` directly, which means buffers can be yanked and pasted between tabs of the same window, but not between windows. This matches the behavior for MacVim out of the box. As a bonus, we now have a well-defined place to put things like search history. And, if someone wanted to abuse it in a hypothetical module, ex history.