Fix singular option

before:
```
$ oil g scaffold monkey name:string description:text --singular -f
	Creating migration: APPPATH/migrations/001_create_monkeys.php (*)
	Creating model: APPPATH/classes/model/monkey.php
	Creating controller: APPPATH/classes/controller/monkey.php
	Creating view: APPPATH/views/monkey/index.php
	Creating view: APPPATH/views/monkey/view.php
	Creating view: APPPATH/views/monkey/create.php
	Creating view: APPPATH/views/monkey/edit.php
	Creating view: APPPATH/views/monkey/_form.php
	Creating view: APPPATH/views/template.php
```

after:
```
$ oil g scaffold monkey name:string description:text --singular -f
	Creating migration: APPPATH/migrations/001_create_monkey.php (*)
	Creating model: APPPATH/classes/model/monkey.php
	Creating controller: APPPATH/classes/controller/monkey.php
	Creating view: APPPATH/views/monkey/index.php
	Creating view: APPPATH/views/monkey/view.php
	Creating view: APPPATH/views/monkey/create.php
	Creating view: APPPATH/views/monkey/edit.php
	Creating view: APPPATH/views/monkey/_form.php
```


before:
```
$ oil g scaffold bbs name:string description:text --singular -f
	Creating migration: APPPATH/migrations/002_create_bbs.php
	Creating model: APPPATH/classes/model/bb.php (*)
	Creating controller: APPPATH/classes/controller/bbs.php
	Creating view: APPPATH/views/bbs/index.php
	Creating view: APPPATH/views/bbs/view.php
	Creating view: APPPATH/views/bbs/create.php
	Creating view: APPPATH/views/bbs/edit.php
	Creating view: APPPATH/views/bbs/_form.php
```

after:
```
$ oil g scaffold bbs name:string description:text --singular -f
	Creating migration: APPPATH/migrations/001_create_bbs.php
	Creating model: APPPATH/classes/model/bbs.php (*)
	Creating controller: APPPATH/classes/controller/bbs.php
	Creating view: APPPATH/views/bbs/index.php
	Creating view: APPPATH/views/bbs/view.php
	Creating view: APPPATH/views/bbs/create.php
	Creating view: APPPATH/views/bbs/edit.php
	Creating view: APPPATH/views/bbs/_form.php
```

Related: https://github.com/fuel/oil/issues/50