expose(:foo, permit: [:bar, :baz])

Add a permit option rather than specifying a method for attributes to permit params.

```ruby
class FooController < ApplicationController
  expose(:foo, attributes: :foo_params)

  private
  def foo_params
    params.require(:foo).permit(:bar, :baz)
  end
end
```
becomes
```ruby
class FooController < ApplicationController
  expose(:foo, permit: [:bar, :baz])
end
```
I'm a noob and couldn't get the specs Inflector mock to play ball..
"no implicit conversion of Symbol into String"

But, the implementation is behaving as I expect.