run_editor: open $VISUAL or $EDITOR at current slide location

[borgand]

This PR maps keyboard shortcut v to open the slide deck in $VISUAL or $EDITOR (or vim).

If the editor is any of vim, joe or nano the cursor is placed at the start of the current slide. For that purpose, slide hash keeps record of line numbers where the slides are defined.

This is useful:

• when reviewing slides to fix typos
• during presentation to add notes etc right into the presentation