Add instance option to append a CSS class for rows

[ghost]

As Rails Admin uses Bootstrap, I wanted to use the table's contextual classes in my lists.
This is an approach to implement that using a method declared in RailsAdmin::Config, it has access to the object being rendered into the row.

Usage

RailsAdmin.config do |config|
config.model Event do
  list do
    row_css_class do
      bindings[:object].mode == 'panic' ? 'danger' : 'success'
    end
end

[mshibuya]

Test needed!

[ghost]

indeed, I'll add them soon, @mshibuya; however, do you find it helpful?
I really don't know if there's a better/cleaner way to implement this.

[mshibuya]

I think this is okay, it's simple and useful 😃

[ghost]

@mshibuya, I will close this PR in favour of #2860.
It will be great if you can give me another review (after almost 1 year, hehe)