Add ActionView::Helpers::TextHelper#current_cycle to return the current cycle for better design options.

Access to the current value returned by cycle() helps in designing tables that have col and row spans beyond the normal 1 row zebra strip. The current_cycle() method uses the new Cycle#current_value method to return just that. The Cycle class was also re-factored a bit to keep things DRY, specifically the methods for getting the next and previous index.

Tests included cover the cases for returning the current cycle when the cycle object has a name, or not, or even more than two items. Tests included also show that calling current_cycle accidently before a cycle object was created is safe to do, just like reset_cycle.

This addition will give designers a broader range of options with cycle. Especially when needing a simple way to get the current cycle in a partial rendered further below the initial cycle() call.