Add `steps` and `current_step_index` in view helper.

Sometime, we need to know `current_step_index` and list of `steps` in view.

For exemple to easily create a breadcrumb :
```ruby
  <ol class="breadcrumb text-center">
    <% steps.each_with_index do |step, index| %>

      <% if index < (current_step_index - 1) %>
        <li><%= link_to I18n.t("wicked.#{step}"), wizard_path(step) %></li>
      <% elsif index == (current_step_index - 1) %>
        <li class="active"><%= link_to I18n.t("wicked.#{step}"), wizard_path(step) %></li>
      <% else %>
        <li><%= I18n.t("wicked.#{step}") %></li>
      <% end %>

    <% end %>
  </ol>
```