Add support for @first, @last and @index

[schuetzm]

Closes #2.

[vincent-psarga]

Ok, it seems there's a slight issues with another PR that was merged previously (allowing non hash instances). So basically the merge! does not work anymore.

The idea I'm checking is:

• saving the "this" value as previously
• using your new methods to store @FIRST, @last and @index

If the each is used on a non-hash object, the special values won't be available, otherwise it works as you meant it.

Does it sound good for you ?

      register_helper('each') do |context, items, block, else_block|
        current_this = context.get('this')
        saved = context.save_special_variables

        if (items.nil? || items.empty?)
          if else_block
            result = else_block.fn(context)
          end
        else
          result = items.each_with_index.map do |item, index|
            context.add_item(:this, item)
            context.add_items :@index => index, :@first => (index == 0), :@last => (index == items.length-1)
            block.fn(context)
          end.join('')
        end

        context.add_item(:this, current_this)
        context.restore_special_variables saved
        result
      end

[vincent-psarga]

That's merged (with some twist to make it a bit more generic for the things you've added).
Thank you for the contribution.