available_users_names also accepts procs

@lcowell It would be super cool if I could specify a Proc as well as a Symbol for my select label names. e.g.

```ruby
# config/initializers/switch_user.rb
config.available_users_names = { :user => proc { |u| "#{u.role_name} - #{u.email}" } }
```

Is this good to merge?