Izk

Improvements:
- Enable vi mode by default
- Don't display a traceback when mashing Ctrl-D on exit confirmation message

Fixes:
- `raw` commands are now usable (reported by hzluyang)

New command:
- `edit`: opens an editor to edit the content of a znode

Improvements:
- If only a path is given to `set`, an editor will be opened to type-in the ZNode data
- Command validation refactoring