In this mode, a new cell is not created after the current cell
is run. Once the cell is run, the current input is cleared, so
it acts just like the terminal.
* Extra enter on FF is fixed by hooking into CodeMirror's
onKeyEvent hook. We now have CodeMirror ignore shift-enter
completely as we handle it ourselves.
* The cell execution logic in notebook.js has been refactored and
the Run All/Selected buttons have been hooked up.