Index page title can be set using a proc

I've implemented functionality that allows you to set dynamic titles on index actions, which should resolve #1324 and #1059 (closed due to inactivity?). The proc is instance eval-ed against the controller as there isn't a resource to apply it to in the index action.

I implemented it because needed to modify titles for belongs_to resources to reference their parent. For example, if an Article belongs to a Brand the article index action would be:

```ruby
index title: proc{ "#{@brand.name}: Articles" } do
  # ...
end
```