Toggle autocomplete on/off at the cell level

[jlewi]

It can be annoying when your working on a given cell where you don't want completions if Foyle is constantly suggesting completions.

One way to address this would be to toggle completions on/off for a given cell and possible let the user control whether the default is on or off for all cells.

We could keep trick of this in cell metadata. If its disabled then we wouldn't generate completions.