Maintain pinned status of tab moved between windows

Hi,

Another PR regarding pinned tabs. If you `:tabattach` to another window, a pinned tab now remains pinned.

I also have a couple of other improvements to `:tabattach`:

1. Focus moved tab. I think this makes more sense. Whenever I use `:tabattach`, I still want the tab to be the current focus (like I would when I move a tab within a window with `<` and `>`).

2. `:tabattach 0` to move tab to a new window (and include "0 new window" in the completion). I don't believe there is another way to do this yet?

I haven't written them yet, but if they are features you would merge, let me know and I will.