Provide for setting html attributes on inner tab element (a or span)

### The goal:

In certain situations, I want to put a class (or perhaps other attributes) on the inner <a> or <span> in my tabs. E.g.:

```html
<ul>
  <li>
    <a class="icon-ok" href="/orders/complete">Completed Orders</a>
  </li>
  <li class="current">
    <span class="icon-cancel">Cancellations</span>
  </li>
</ul>
```

Currently, the only way to do that is to create a custom builder. Would it be worth providing this capability out of the box? The API proposed in this pull request would allow the above to be generated with the following:

```html
<%= tabs_tag do |tab| %>
  <%= tab.completed "Completed", completed_path, inner_options: { class: "icon-ok" }
  <%= tab.cancellations "Cancellations", cancellations_path, inner_options: { class: "icon-cancel" }
<% end %>
```

I'm not sure about the name "inner_options", but at least the approach doesn't pollute the pre-existing API. It's just available for the odd people who need it.

I realize I can allow for this with a custom builder. However, that seems like a lot of trouble for just getting basic control of my markup. I thought I would throw this proposal out there to start a conversation.