Add plural names as Placeholder.

- Previous
```
placeholder :password do 
  match /valid/ do 
    'foo@example.com'
  end
end
placeholder :confirmation_password do 
  match /valid/ do 
    'foo@example.com'
  end
end
```
- After
```
placeholder :password, :confirmation_password do 
  match /valid/ do 
    'foo@example.com'
  end
end
```