Adding support for running, editing and previewing cells.

[amtoine]

Hello there!

This might sound like issue #4, but I really would like to show my interest in such features for this plugin.

Running cells: to be able to run notebooks without jupyter.
Editing cells: to be able to change an existing notebook or create a new one from scratch, inside neovim, leveraging the vim keybindings and the neovim config.
Previewing cells: I do not even know if it is even possible, maybe with ueberzug. But having something like a window on the side of the notebook editing window, to preview images and, more generally, markdown cells, would be super nice.

Cheers guys ;)