VROOM navigation/presentation commands

* Hit <SPACE> to move forward.
* Hit <BACKSPACE> to go backwards.
* Hit 'Q' to quit.
* Hit 'RR' to execute the current page as program and show STDOUT.
* Hit 'QQ' to quit.
* Hit 'EE' to edit a file whose name is under the cursor.
* Hit 'VV' to launch Vroom on the current buffer.