Jupyterlab-vim

* Requires JupyterLab 0.32 (Beta 2)
* ctrl-g shows function signature (36)

* center cell command (33)
* `-` splits cell in vim command mode (33)
* ctrl-c copies instead of leaving insert mode if you aren't on a mac (34)

* add moving cell commands
* Add delete/yank/paste cell commands (28)
* select first and last cell shortcuts (24)
* change to focus selector to fix debug prompt select

* Ctrl-J and Ctrl-K execute markdown when leaving the cell.

* Added Command/Ctrl-{1,2,3} to switch cell mode to code, markdown and raw.

* Ex commands ':q' and ':quit' leave vim mode