Use File.basename instead of sub()
Adopt new Vundle interface
Fix typo
Separate steps instead of changes
Update comment
Add option --list for command clean to list all unused plugins
Correct syntax in help message for command find
Refactor for prettier output messages

* Include Helpers module.
* Use say, say_ok, say_warning, say_error for prettier outputs.
* enable_paging when listing all plugins.
* Update help message for `clean` command: Plugin name does not have to
be exact.