Add :Onoff command
Update usage help in documentation and readme
Remove <Plug> mappings

Use :Onoff command instead.
Abbreviate :Onoff command to :onoff
Abbreviate command only when it's the first word on command line
Rewrite readme

- Mention :Onoff command in readme
- Remove FAQ section (any reasons to keep it?)
Add autocompletion for available boolean options
Merge pull request #4 from vitalk/onoff-command

Add :Onoff command.