Rubberband

This extension allows to select cells in a notebook using a rubberband style selection.
Press `shift + click mouse` and drag the rubberband over the desired cells:
This marks all cells touching the rubberband and adds `cell.metadata.selected=true`.
Press `ctrl + shift + click mouse` and drag the rubberband keeps already selected cells selected.

Works only for cells that are currently visible in the browser. Not sure how an UI for scrolling while dragging the rubberband  should work.