Add :Onoff command

- [x] Define :Onoff command
- [x] Command abbreviation
- [x] Only one public interface (:Onoff <option>)
- [x] Update help documentation (:help :Onoff)
- [x] Add usage example in readme
- [x] Command autocompletion for available boolean options
- [x] Safely escape file name in compfunc
- [x] Fallback if file is not exist